Inheritance

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Genetic Inheritance & Genetic Crossing

Gametes are haploid sex cells formed by meiosis, containing one copy of each chromosome and therefore one allele of each gene.
The egg cell is larger than the sperm as it contains food to nourish the embryo; the sperm cell contains many mitochondria to release energy for movement.
Fertilisation is the fusion of gamete nuclei to form a diploid zygote, restoring two alleles of each gene and creating genetic variation in offspring.
Mendel carried out breeding experiments on pea plants, transferring pollen manually to eliminate uncertainty about parentage.
Mendel found that the first generation (F1) all showed one parental characteristic, while the second generation (F2) showed both characteristics in a 3:1 ratio.
A monohybrid cross starts with pure-breeding (homozygous) parents showing different phenotypes; a Punnett grid predicts the probability of offspring genotypes and phenotypes.
To construct a Punnett grid: write parental phenotypes and genotypes, list possible gametes, place gametes on axes, combine them in boxes, and list genotype and phenotype ratios.

A Punnett square

Inheritance: Terminology

A gene is a length of DNA on a chromosome that codes for a particular characteristic by coding for a specific protein.
Alleles are variations of the same gene; every individual has two alleles of each gene, one inherited from each parent.
The genotype is the combination of alleles an organism inherits; if the two alleles are the same it is homozygous, if different it is heterozygous.
The phenotype is the observable characteristics of an organism, determined by genotype alone (e.g. blood group), environment alone (e.g. scars), or an interaction of both (e.g. height, skin colour).
A dominant allele needs to be inherited from only one parent to be expressed; a recessive allele must be inherited from both parents to be expressed.
Co-dominant alleles are both expressed to an equal extent in the phenotype, e.g. speckled chickens (C^W C^B).
Incomplete dominance occurs when both alleles are partially expressed, producing an intermediate phenotype, e.g. pink flowers in the marvel of Peru (C^W C^R).

Inheriting Alleles

Phenotypic plasticity is the idea that although genotype remains fixed, phenotype can vary due to internal or external environmental influences on gene expression.
Environmental stimuli such as light, chemicals, drugs and hormones can affect levels of regulatory proteins or transcription factors.
UV radiation activates enzymes that increase melanin production, leading to skin pigmentation.
Temperature can influence gene expression: the Himalayan rabbit's pigmentation gene is inactive above 35°C but active between 15°C and 25°C, causing black fur on cooler body parts.
Phenylketonuria (PKU) is caused by a recessive allele on an autosome; a mutation in the PAH gene results in a non-functional phenylalanine hydroxylase enzyme, causing a build-up of phenylalanine.
Two heterozygous carriers of PKU have a 25% chance of a child with PKU and a 75% chance of a child without PKU.
Multiple alleles exist for some genes (e.g. ABO blood groups), but a diploid individual still inherits only two; alleles differ by one or a few bases called SNPs (Single Nucleotide Polymorphisms).

Sex Determination

Sex is determined by an entire chromosome pair: females are XX and males are XY (in mammals).
All other chromosomes (pairs 1–22 in humans) are autosomes and do not determine sex.
The father determines the sex of the child because only he can pass on a Y chromosome; half of his sperm carry X and half carry Y.
The X chromosome is larger than the Y and carries around 16 times more genes; non-sex traits such as blood clotting factors are coded on the X but not the Y.
The SRY gene on the Y chromosome is involved in testes development and testosterone production in male embryos.
Sex-linked disorders such as haemophilia are caused by genes on one sex chromosome; haemophilia is caused by a recessive allele for factor VIII on the X chromosome.
Females can be normal, carriers, or affected by haemophilia; males have only one copy of the gene so are affected if they inherit the recessive allele.

Pedigree Charts

Pedigree charts trace the inheritance of a characteristic through generations; males are squares and females are circles, with affected individuals shaded.
Horizontal lines between individuals show they have produced children, linked underneath; Roman numerals may indicate generations, with the eldest child on the left.
A recessive condition can be identified if it skips a generation or if unaffected parents have an affected child (both parents must be carriers).
It is unlikely to be sex-linked if both males and females are affected.
The genotype of an affected individual is homozygous recessive; carrier parents are heterozygous.
Unaffected individuals in a pedigree may be homozygous dominant or heterozygous.
Marriage between close relatives is prohibited in many countries because it increases the chance that both parents carry harmful recessive alleles, raising the risk of genetic disease in offspring.

A family pedigree chart

Continuous Variation: Skills

Discrete variation falls into clear-cut categories with no overlap, e.g. blood groups (A, B, AB, O with Rhesus factor).
Continuous variation occurs when two or more genes affect a characteristic, producing a range of values between two extremes, e.g. height, birth mass, skin colour.
Continuous variation is caused by an interaction between genetics and the environment: Phenotype = genotype + environment.
Polygenes are a large number of genes that have a combined additive effect on the phenotype.
Continuous variation in a population often displays a normal distribution (bell-shaped curve).
Box plots (box-and-whisker diagrams) split data into quartiles and require five values: lowest, first quartile, median, third quartile, highest.
The box represents the interquartile range (middle 50% of data); whiskers represent the lowest and highest 25%.
Outliers are data points more than 1.5 × the interquartile range above the third quartile or below the first quartile, plotted separately.

Continuous variation

Dihybrid Crosses & Unlinked Genes

Unlinked genes are on separate chromosomes and segregate independently during meiosis.
Assortment refers to the independent alignment of bivalents in metaphase I; segregation refers to which pole whole chromosomes are pulled to in anaphase I.
Mendel's Law of Independent Assortment states that alleles of different genes are inherited independently of one another.
Linked genes are on the same chromosome and tend to be inherited together.
Dihybrid crosses show the inheritance of two different characteristics; genotypes are written with the two alleles of one gene followed by the two alleles of the other, e.g. YyGg.
In a dihybrid cross between two double heterozygotes (e.g. BbEe × BbEe), the expected phenotypic ratio is 9:3:3:1.
Gametes for dihybrid crosses can be determined using the FOIL method (First, Outside, Inside, Last).

Statistical Analysis of Dihybrid Crosses

A chi-squared test compares observed phenotypic ratios with expected ratios to determine whether differences are due to chance.
The calculated chi-squared value is compared with a critical value at a 5% probability level.
If the chi-squared value is less than the critical value, the results are not significantly different from the expected ratio; the probability that results are due to chance is high.
If the chi-squared value is greater than the critical value, the results are significantly different from the expected ratio; the probability that results are due to chance is low.
The critical value depends on the degrees of freedom, which is the number of categories minus one.

Genes & Polypeptides: Skills

A gene is a section of DNA that codes for one polypeptide; the number of genes a species has is not directly proportional to its size or complexity.
A gene may be found at the same locus in different individuals; alleles of the same gene occupy the same locus.
New alleles can arise through mutation; there are not always two alleles of every gene (some genes have multiple alleles).
Not every eukaryotic cell contains two alleles of every gene; gametes are haploid and contain only one allele of each gene.
Online gene databases can be used to find the locus of a human gene, the chromosome on which a gene is located, and evolutionary relationships between organisms; they are not restricted to scientists with special access.

Gene Linkage & Identifying Recombinants

Linked genes are located on the same chromosome and do not assort independently, so they tend to be inherited together.
In a test cross, if two genes are linked, the offspring mostly show parental combinations rather than the expected 1:1:1:1 ratio.
Recombinants are offspring with combinations of alleles different from those of the parents, produced by crossing over during prophase I of meiosis.
Crossing over between homologous chromosomes exchanges sections of DNA, creating new combinations of linked alleles.
The frequency of recombinants depends on the distance between the genes on the chromosome; the further apart they are, the more likely crossing over occurs between them.
When writing linked alleles, the combination on each chromosome should be shown, e.g. GH/gh indicates G and H are linked on one chromosome and g and h on the other.
A test cross with a homozygous recessive individual can reveal whether genes are linked by the ratio of parental to recombinant offspring.

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1.Which of the following is not an example of how the external environment can influence gene expression?
Easy
- AFur colouration of Siamese cats
- BPetal colour of snapdragons
- CSkin colouration after sun exposure
- DSea turtle eggs hatching as females if incubated at 30°C
2.Which option correctly completes the sentence: 'Multiple alleles are present _________.'
Easy
- AAt the same locus on homologous chromosomes
- BAt different loci on different chromosomes
- CAt different loci on the same chromosome
- DOn sister chromatids
3.The diploid chromosome number of a koala (Phascolarctos cinereus) is 16. How many autosomes are there in a koala's egg cell?
Medium
- A7
- B8
- C14
- D32
4.Two pea plants are heterozygous for two traits, pod and flower colour. The dominant trait for flower colour is purple and the recessive is white. The dominant trait for pod colour is green and the recessive is yellow. If they were to breed, what would be the ratio of their offspring's traits? The ratio is as follows: purple green : purple yellow : white green : white yellow.
Medium
- A2 : 1 : 1 : 2
- B1 : 1 : 1 : 1
- C9 : 3 : 3 : 1
- D3 : 3 : 1 : 1
5.In a newt, normal tail length is dominant to short tail length and green scales are dominant to white scales. A heterozygous normal tailed newt with green scales was crossed with a short tailed newt with white scales. A large number of offspring were produced. They were either normal tailed with green scales or short tailed with white scales in equal number. What is the most likely cause of this pattern?
Medium
- AThe genes are codominant
- BCrossing over has occurred
- CThe two genes are linked
- DThe traits are polygenic
6.Hair colour is a trait controlled by polygenic inheritance. Which statement is correct?
Easy
- AHair colour shows discontinuous variation
- BIndividuals show a wide range of phenotypes for hair colour
- CNo two people have the same hair colour
- DChildren always have the same hair colour as one of their parents
7.Which of the following statements correctly describes co-dominance?
Medium
- AThe alleles are both expressed to an equal extent in the phenotype.
- BTwo alleles are 'blended together'.
- COne allele is dominant over the other.
- DNeither allele is expressed in the phenotype.
8.Which option correctly completes the sentence: 'The segregation of alleles occurs during _________ in eukaryotic organisms.'
Easy
- AMitosis
- BBinary fission
- CMeiosis
- DDNA replication before cell division

Soal lainnya

9.Which of the following statements is a feature of X-linked genetic diseases in humans?
Medium
- AFemales can’t inherit the defective gene as they don’t possess a Y chromosome.
- BFemales can either be normal, a carrier or have the disease.
- CMales can be a carrier for the defective allele.
- DFemales can only be heterozygous for the defective gene.
10.Identical twins have the same DNA (genotype) but often display many observable physical differences (phenotype). Studies on twins have shown that one twin can be more susceptible to diseases like cancer or diabetes. Which of the following statements best explains these differences between identical twins?
Medium
- AOne twin is more prone to random mutations of DNA.
- BEpigenetic changes can affect the structure of proteins.
- COne twin inherited epigenetic modifications from a parent.
- DAn organism's environment can influence gene expression patterns.
11.Which of the following statements about genes is not correct?
Medium
- AA gene may be found at a different locus in different individuals.
- BA gene is a section of DNA that codes for one polypeptide.
- CGenes are heritable factors.
- DThe number of genes that a species has is not directly proportional to the size or complexity of the organism.
12.Which of the following statements about alleles are correct? 1. New alleles can arise through mutation. 2. There are two different versions, or alleles, of every gene. 3. Every eukaryotic cell contains two alleles of every gene.
Medium
- AI only
- BI and II only
- CI and III only
- DII and III only
13.Place the steps of constructing a Punnett grid in the correct order.
Medium
- Write down the parental phenotypes and genotypes
- Write down all the possible gamete genotypes that each parent could produce
- Place each parental genotype against one axis of a 2 x 2 Punnett grid
- Combine the gametes in each box to give the possible genotypes of the offspring
- List the phenotype and genotype ratios for the offspring
14.Match each genetic term with its correct definition.
Medium
- Genotype
- Phenotype
- Homozygous
- Heterozygous
- The combination of alleles that an individual organism inherits
- The observable characteristics of an organism
- Having two identical alleles at a locus
- Having two different alleles at a locus
15.Which of the following statements about codominance and incomplete dominance are correct? (select all that apply)
Medium
- AIn codominance, both alleles are expressed to an equal extent in the phenotype.
- BIn incomplete dominance, the phenotype is a blend or intermediate between the two alleles.
- CSpeckled chickens are an example of incomplete dominance.
- DThe marvel of Peru (Mirabilis jalapa) with pink flowers is an example of codominance.
- EIn codominance, one allele is completely dominant over the other.
16.Which of the following are features of X-linked genetic diseases in humans? (select all that apply)
Hard
- AFemales can be normal, carriers, or have the disease.
- BMales can be carriers for the defective allele.
- CMales are affected more often than females.
- DFemales can only be heterozygous for the defective gene.
- EThe defective gene is carried on the X chromosome, so males have only one copy.
17.In a dihybrid cross between two heterozygous parents for two unlinked genes (e.g. BbEe x BbEe), what is the expected phenotypic ratio?
Medium
- A9:3:3:1
- B1:1:1:1
- C3:1
- D1:2:1
18.Two genes are said to be linked. What does the term 'linked' mean?
Medium
- ATwo genes are both at the same loci on a chromosome
- BTwo genes both occupy the same chromosome
- CThey are both alleles of the same gene
- DThe phenotypes of the genes are both influenced by the gender of their owner
19.In humans, the father determines the sex of the offspring because only sperm cells can carry either an X or a Y chromosome.
Easy
True or false?
20.Which of the following are examples of continuous variation? (select all that apply)
Medium
- AHuman height
- BBlood group
- CSkin colour
- DBirth mass
- EAbility to roll the tongue
21.A gamete contains only one allele of each gene. Which statement explains why?
Easy
- AGametes are haploid, so they contain one chromosome from each homologous pair and therefore one allele of each gene
- BGametes are diploid, so they contain two alleles of each gene
- CGametes contain only the dominant allele of each gene
- DGametes contain only the recessive allele of each gene
22.In a plant species, flower colour is controlled by a single gene with two alleles: R (red) and r (white). A heterozygous red-flowered plant is crossed with a white-flowered plant. What proportion of the offspring is expected to have white flowers?
Medium
- A0%
- B25%
- C50%
- D100%
23.In humans, the gene for haemophilia is carried on the X chromosome. A woman who is a carrier for haemophilia marries a man who does not have haemophilia. What is the probability that a son born to this couple will have haemophilia?
Medium
- A0%
- B25%
- C50%
- D100%
24.A chi-squared test is used to analyse the results of a dihybrid cross. The calculated chi-squared value is 3.5. The critical value at the 5% probability level is 7.8. What conclusion can be drawn?
Medium
- AThe observed results are significantly different from the expected ratio; the genes are likely linked
- BThere is no significant difference between observed and expected results; the genes are likely unlinked
- CThe results are due to chance; the genes are likely linked
- DThe results are not due to chance; the genes are likely unlinked
25.Two genes are located close together on the same chromosome. Which statement best describes their inheritance?
Medium
- AThey assort independently during meiosis
- BThey are always inherited together
- CThey tend to be inherited together, but crossing over can separate them
- DThey are always separated during meiosis
26.Which of the following statements about sex determination in humans are correct? (select all that apply)
Medium
- AFemales have two X chromosomes and males have one X and one Y chromosome
- BThe father determines the sex of the offspring because only sperm can carry either an X or a Y chromosome
- CThe mother determines the sex of the offspring because she contributes an X chromosome
- DThe SRY gene on the Y chromosome is involved in the development of testes
27.In humans, the X chromosome carries more genes than the Y chromosome.
Easy
True or false?
28.Match each genetic term with its correct description.
Medium
- Homozygous
- Heterozygous
- Phenotype
- Genotype
- The observable characteristics of an organism
- The combination of alleles an organism inherits
- Having two identical alleles of a gene
- Having two different alleles of a gene
29.A test cross is carried out to determine the genotype of a fruit fly with a brown body (brown body is dominant to black body). Which genotype should the brown-bodied fly be crossed with?
Medium
- AHomozygous recessive
- BHeterozygous
- CHomozygous dominant
- DA fly with a black body that is heterozygous
30.In a dihybrid cross between two double-heterozygous fruit flies (MmRr × MmRr), 160 offspring are produced. The genes are unlinked. What is the expected number of offspring with red eyes and a black body? (M = brown body dominant to m = black body; R = red eyes dominant to r = white eyes.)
Medium
- A90
- B120
- C30
- D10
31.A chi-squared test is used to analyse the results of a dihybrid cross. The calculated chi-squared value is 3.5. The critical value at the 5% probability level is 7.8. Which conclusion is correct?
Hard
- AThe null hypothesis is rejected; the results are not due to chance.
- BThe null hypothesis is accepted; the results are due to chance.
- CThe null hypothesis is rejected; the results are due to chance.
- DThe null hypothesis is accepted; the results are not due to chance.
32.A heterozygous normal-tailed newt with green scales is crossed with a short-tailed newt with white scales. The offspring are either normal-tailed with green scales or short-tailed with white scales in equal numbers. What is the most likely explanation?
Medium
- AThe genes are codominant.
- BCrossing over has occurred.
- CThe two genes are linked.
- DThe traits are polygenic.
33.Two genes are denoted by the dominant alleles G and H, with recessive alleles g and h. Which notation correctly shows that the genes are linked in the coupling (cis) arrangement?
Medium
- AGgHh
- BGH/gh
- CG/H; g/h
34.In a dihybrid cross between two heterozygous parents (BbEe × BbEe) for two unlinked genes, what is the expected phenotypic ratio?
Medium
- A1:1:1:1
- B3:1
- C9:3:3:1
- D1:2:1
35.Which of the following statements about linked genes are correct? (select all that apply)
Medium
- ALinked genes are located on the same chromosome.
- BLinked genes always assort independently.
- CCrossing over can separate linked alleles.
- DLinked genes are on homologous chromosomes.
- EThe closer two genes are, the more likely they are inherited together.
36.In a chi-squared test, if the calculated value is greater than the critical value, the null hypothesis is rejected.
Easy
True or false?
37.Match each term related to experimental genetics with its correct description.
Medium
- Test cross
- Chi-squared test
- Punnett grid
- Pedigree chart
- A cross with a homozygous recessive individual to reveal an unknown genotype
- A statistical test comparing observed and expected ratios
- A diagram used to predict the probability of offspring genotypes
- A family tree showing the inheritance of a trait across generations
38.Which of the following is the correct definition of an allele?
Easy
- AA different form of the same gene
- BA section of DNA that codes for a polypeptide
- CThe observable characteristics of an organism
- DThe position of a gene on a chromosome
39.Which option correctly completes the sentence below? Multiple alleles are present _________.
Easy
- AAt the same locus on homologous chromosomes
- BAt different loci on different chromosomes
- CAt different loci on the same chromosome
- DOn sister chromatids
40.Which of the following statements about alleles are correct? (select all that apply)
Medium
- ANew alleles can arise through mutation.
- BThere are two different versions, or alleles, of every gene.
- CEvery eukaryotic cell contains two alleles of every gene.
- DAlleles of the same gene occupy the same locus on homologous chromosomes.
- EA diploid individual can inherit more than two alleles of a gene.
41.In Drosophila, brown body (M) is dominant to black body (m) and red eyes (R) are dominant to white eyes (r). Two double-heterozygous flies are crossed and produce 160 offspring. Assuming the genes are unlinked, how many offspring would be expected to have red eyes and a black body?
Hard
42.Complete the sentence about blood group inheritance.
Medium
The ABO blood group system is an example of co-dominance with multiple alleles, where the ____ allele codes for antigen A and the ____ allele codes for antigen B.
43.In a dihybrid cross between two heterozygous individuals for two unlinked genes (BbEe x BbEe), what is the expected percentage of offspring that are homozygous recessive for both genes (bbee)?
Medium
44.Complete the sentence about Mendel's experiments.
Easy
Mendel studied inheritance in ____ plants and found that characteristics were inherited in a predictable pattern, with a ____ ratio in the second generation.
45.What is the term for the observable characteristics of an organism?
Medium
46.A koala's diploid chromosome number is 16. How many autosomes are present in a koala's egg cell?
Medium
47.In a population, the frequency of the IO allele is 0.72. The remaining two alleles (IA and IB) are equally abundant. What is the frequency of the IB allele?
Medium
48.In a dihybrid cross between two heterozygous individuals for two unlinked genes (BbEe x BbEe), what is the expected percentage of offspring that are homozygous recessive for both genes?
Medium
49.In humans, the gene for haemophilia is carried on the X chromosome. A woman who is a carrier for haemophilia marries a man who does not have haemophilia. What is the probability that a son will have haemophilia?
Medium
50.Complete the sentence about continuous variation.
Medium
Continuous variation is controlled by many genes, known as ____, and is also influenced by the ____.
51.What is the name of the specific gene on the Y chromosome that is involved in the development of testes in male embryos?
Easy
52.A chi-squared test is used to analyse the results of a dihybrid cross. The calculated chi-squared value is 3.5. The critical value at the 5% probability level is 7.8. What is the probability that the results are due to chance? (Give your answer as a percentage, to the nearest whole number.)
Medium
53.Explain why linked genes do not assort independently during meiosis.
Hard
54.In a pedigree chart, two unaffected parents have a child affected by a genetic disorder. Deduce the most likely mode of inheritance and explain your reasoning.
Hard
55.Explain how a pedigree chart can be used to deduce that a genetic disorder is caused by a recessive allele.
Hard
56.In Drosophila, brown body (M) is dominant to black body (m) and red eyes (R) are dominant to white eyes (r). Two double-heterozygous flies are crossed and produce 160 offspring. Assuming the genes are unlinked, how many offspring would be expected to have red eyes and a black body?
Hard
- A30 offspring
- B36 offspring
- C33 offspring
- D27 offspring
57.In a dihybrid cross between two heterozygous individuals for two unlinked genes (BbEe x BbEe), what is the expected percentage of offspring that are homozygous recessive for both genes (bbee)?
Medium
- A6.25 %
- B5.25 %
- C8.25 %
- D7.25 %
58.A koala's diploid chromosome number is 16. How many autosomes are present in a koala's egg cell?
Medium
- A6
- B9
- C7
- D8
59.In a population, the frequency of the IO allele is 0.72. The remaining two alleles (IA and IB) are equally abundant. What is the frequency of the IB allele?
Medium
- A2.14
- B0.14
- C1.14
- D-0.86
60.In a dihybrid cross between two heterozygous individuals for two unlinked genes (BbEe x BbEe), what is the expected percentage of offspring that are homozygous recessive for both genes?
Medium
- A6.25 %
- B5.25 %
- C7.25 %
- D8.25 %
61.In humans, the gene for haemophilia is carried on the X chromosome. A woman who is a carrier for haemophilia marries a man who does not have haemophilia. What is the probability that a son will have haemophilia?
Medium
- A55 %
- B60 %
- C50 %
- D45 %
62.A chi-squared test is used to analyse the results of a dihybrid cross. The calculated chi-squared value is 3.5. The critical value at the 5% probability level is 7.8. What is the probability that the results are due to chance? (Give your answer as a percentage, to the nearest whole number.)
Medium
- A6 %
- B4 %
- C5 %
- D7 %

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