Inheritance

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Gametes and Fertilisation

Gametes are the sex cells of an organism, such as sperm and egg cells in humans.
The egg cell is larger than the sperm because most of its space contains food to nourish a growing embryo.
The sperm cell contains many mitochondria to release energy for its motion.
Gametes are formed during meiosis and are haploid, meaning they have one copy of each chromosome (23 single chromosomes in humans).
Fertilisation is the fusion of gamete nuclei to form a zygote (fertilised egg cell).
The zygote is diploid, containing two chromosomes of each type (46 chromosomes in humans) and therefore two alleles of each gene.
If the two alleles for a gene are the same, the genotype is homozygous; if different, it is heterozygous.

Genetic Crosses and Mendel

Gregor Mendel was an Austrian monk who carried out breeding experiments on pea plants in the mid-19th century.
Mendel transferred pollen from one pea plant to another, eliminating uncertainty about which pollen fertilised each plant.
He studied characteristics such as plant height, flower colour, and seed coat smoothness.
Mendel found that characteristics were inherited in predictable patterns: the first generation all showed one parental characteristic, and the second generation showed both in a 3:1 ratio.
Mendel discovered 'units of inheritance' (now called genes) and that some genes are dominant and some recessive.
A monohybrid cross starts with pure-breeding (homozygous) parents displaying different phenotypes; this is the P generation.
A Punnett grid predicts the probability of offspring displaying a certain genotype or phenotype.

A Punnett square

Inheritance Terminology

A gene is a short length of DNA on a chromosome that codes for a particular characteristic by coding for a specific protein.
Alleles are different forms of the same gene; individuals inherit two alleles for each gene, one from each parent.
The genotype is the combination of alleles an organism inherits.
The phenotype is the observable characteristics of an organism.
Phenotype can be determined by genotype only (e.g., blood group), environment only (e.g., scars, accent), or an interaction of both (e.g., height, skin colour).
A dominant allele only needs to be inherited from one parent to be expressed in the phenotype.
A recessive allele must be inherited from both parents to be expressed; if only one copy is present, it remains hidden.

Codominance and Incomplete Dominance

Codominant alleles both have an equal effect on the phenotype and are both expressed.
In speckled chickens, the alleles for white (C^W) and black (C^B) feather colour are codominant; C^W C^B chickens have both white and black feathers.
Incomplete dominance occurs when both alleles are partially expressed, producing an intermediate phenotype.
In the marvel of Peru (Mirabilis jalapa), a cross between white (C^W C^W) and red (C^R C^R) flowers produces pink (C^W C^R) flowers.
In incomplete dominance, the heterozygote shows a blend of the two parental phenotypes.

Phenotypic Plasticity and Recessive Inheritance

Phenotypic plasticity is the idea that although genotype remains fixed, phenotype can vary during an organism's lifetime due to environmental influences.
Environmental stimuli such as light, chemicals, drugs, and hormones can affect gene expression patterns.
In the Himalayan rabbit, the gene for fur pigmentation is inactive above 35°C but active between 15°C and 25°C, causing black fur on cooler body parts (ears, feet, nose).
Phenylketonuria (PKU) is an inherited condition caused by a recessive allele on an autosome.
PKU is caused by a mutation in the PAH gene, leading to a non-functional enzyme (phenylalanine hydroxylase) and a build-up of phenylalanine.
Two heterozygous carriers of the PKU allele have a 25% chance of having a child with PKU and a 75% chance of having a child without PKU.
This pattern of inheritance is the same for other autosomal recessive conditions, such as cystic fibrosis.

Multiple Alleles and Blood Groups

Many genes have more than two alleles, but a diploid individual still only inherits two of the possible alleles.
Alleles differ from each other by one or a few bases; these positions are called Single Nucleotide Polymorphisms (SNPs).
The ABO blood group system is controlled by a single gene with three alleles: I^A, I^B, and i.
I^A and I^B are codominant, and both are dominant to the recessive allele i.
I^A produces antigen A, I^B produces antigen B, and i produces no antigens on red blood cells.
Genotypes and phenotypes: I^A I^A or I^A i = blood group A; I^B I^B or I^B i = blood group B; I^A I^B = blood group AB; ii = blood group O.
Blood transfusions require matching blood groups to avoid immune responses that can cause clotting and serious illness.

Sex Determination

Sex is determined by an entire chromosome pair: females have XX and males have XY (pair 23 in humans).
All other chromosomes (pairs 1–22) 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 a father's sperm carry an X chromosome and half carry a Y chromosome.
The chromosome carried by the sperm that fertilises the egg determines the sex of the child.
The X chromosome is larger than the Y and carries around 16 times more genes; the Y chromosome carries the SRY gene involved in testes development and testosterone production.

Sex-Linked Disorders: Haemophilia

Sex-linked genes are present on one sex chromosome but not the other; if on the X chromosome, males (XY) have one copy and females (XX) have two.
Females can be normal, carriers, or have the disease; males can only be normal or have the disease.
Haemophilia is a sex-linked disease caused by a recessive allele (f) on the X chromosome that results in a lack of factor VIII, a protein needed for blood clotting.
Males with the recessive allele f will have haemophilia because they have only one copy of the gene.
Females can be heterozygous (X^F X^f) and act as carriers without having the condition.
A cross between a carrier female (X^F X^f) and a normal male (X^F Y) gives a 1:1:1:1 ratio of normal female : carrier female : normal male : male with haemophilia.

Punnett square for a sex-linked condition

Pedigree Charts

Pedigree charts trace the inheritance of a characteristic through generations of a family.
Males are represented by squares and females by circles; affected individuals are shaded or cross-hatched.
Horizontal lines between males and females indicate they have produced children, who are linked underneath.
Roman numerals may indicate generations, and the eldest child is on the left in each generation.
If two unaffected parents have an affected child, the condition must be caused by a recessive allele, and both parents are carriers.
If both males and females are affected, the condition is unlikely to be sex-linked.
Marriage between close relatives is discouraged because it increases the chance that both parents carry harmful recessive alleles, raising the risk of genetic disorders in offspring.

A family pedigree chart

Continuous Variation

Discrete variation falls into clear-cut categories with no overlap, such as blood group (8 distinct groups with Rhesus factor).
Continuous variation occurs when two or more genes affect the final characteristic, producing a range of values between two extremes.
Examples include human height, birth mass, and skin colour.
Continuous variation is caused by an interaction between genetics and the environment: phenotype = genotype + environment.
At the genetic level, different alleles at a single locus have small effects, and different genes can have additive effects; many genes with a combined effect are called polygenes.
Continuous variation in a population often displays a normal distribution (bell-shaped curve).
Box plots (box-and-whisker diagrams) display the lowest value, first quartile, median, third quartile, and highest value; the box represents the interquartile range (middle 50% of data).
Outliers are data points more than 1.5 × the interquartile range above the third quartile or below the first quartile.

Continuous variation

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1.What is the role of factor VIII in humans?
Easy
- AIt helps blood to clot.
- BIt carries oxygen in the blood.
- CIt digests proteins in the stomach.
- DIt regulates blood sugar levels.
2.State the scientific term used for a characteristic coded for by several genes that work in combination to produce the phenotype.
Easy
- APolygenic
- BCodominant
- CHomozygous
- DSex-linked
3.What is meant by the term 'dominant' in the context of alleles?
Easy
- AAn allele that is expressed in the phenotype even if only one copy is present.
- BAn allele that is only expressed when two copies are present.
- CAn allele that is always inherited from the mother.
- DAn allele that is located on the Y chromosome.
4.In snapdragon plants, a cross between a red and white flowered plant results in pink flowered offspring due to codominance. In Mendel's pea plants, a cross between a red-flowered plant and a white-flowered plant did not result in any pink flowered plants. Suggest why.
Medium
- AIn pea plants, the red allele is completely dominant over the white allele.
- BIn pea plants, the red and white alleles are codominant.
- CIn pea plants, flower colour is determined by multiple alleles.
- DIn pea plants, flower colour is a polygenic trait.
5.In a genetic diagram where H denotes the dominant allele for Huntington's disease and h denotes the recessive allele, what is the meaning of the genotype Hh?
Easy
- AThe individual has one dominant and one recessive allele (heterozygous).
- BThe individual has two dominant alleles (homozygous dominant).
- CThe individual has two recessive alleles (homozygous recessive).
- DThe individual has no alleles for the gene.
6.What phenotype will an individual with genotype Hh display for Huntington's disease?
Easy
- AThey will have Huntington's disease.
- BThey will not have Huntington's disease.
- CThey will be a carrier but not affected.
- DThey will have a mild form of the disease.
7.Which of the following statements about gametes are correct? (select all that apply)
Medium
- AGametes are haploid.
- BGametes are produced by meiosis.
- CGametes contain two copies of each chromosome.
- DThe sperm cell contains many mitochondria.
- EThe egg cell is smaller than the sperm cell.
8.A recessive allele needs to be inherited from both parents in order for the characteristic to be expressed in the phenotype.
Easy
True or false?

Plus de questions

9.Match each genetic term with its correct definition.
Medium
- Genotype
- Phenotype
- Homozygous
- Heterozygous
- The observable characteristics of an organism
- The combination of alleles inherited
- Having two identical alleles of a gene
- Having two different alleles of a gene
10.Place the steps of constructing a Punnett grid in the correct order.
Medium
- Write down the parental phenotypes and genotypes.
- Write down all possible gamete genotypes for each parent.
- Place each parental genotype against one axis of a Punnett grid.
- Combine the gametes in each box to give offspring genotypes.
- List the phenotype and genotype ratios for the offspring.
11.Which of the following statements about sex determination in humans are correct? (select all that apply)
Medium
- AA father can pass on either an X or a Y chromosome to his offspring.
- BThe mother's egg cell determines the sex of the child.
- CFemales have two X chromosomes, while males have one X and one Y chromosome.
- DThe Y chromosome carries the SRY gene, which is involved in the development of testes.
- EAll chromosomes in humans are autosomes.
12.Match each genetic term with its correct description.
Medium
- Allele
- Genotype
- Phenotype
- Homozygous
- The combination of alleles an organism inherits
- A different form of a gene
- The observable characteristics of an organism
- Having two identical alleles of a gene
13.Which of the following are examples of continuous variation? (select all that apply)
Medium
- AHuman height
- BABO blood group
- CSkin colour
- DBirth mass
- ESex in humans
14.In a pedigree chart, males are represented by squares and females by circles.
Easy
True or false?
15.A couple has a child with blood group AB. The father has blood group A. Which of the following could be the mother's blood group?
Medium
- AA only
- BB only
- CAB only
- DB or AB
16.Which of the following statements about codominance are correct? (select all that apply)
Medium
- ABoth alleles are expressed equally in the phenotype.
- BThe phenotype is a blend of the two alleles.
- CAn example is the AB blood group in humans.
- DIt is the same as incomplete dominance.
- ESpeckled chickens are an example.
17.Match each genetic condition with its correct description.
Medium
- Phenylketonuria (PKU)
- Haemophilia
- Huntington's disease
- Caused by a dominant allele
- Caused by a recessive allele on an autosome
- Sex-linked recessive condition
18.Arrange the following steps in the order they occur during Mendel's pea plant experiments.
Medium
- Transfer pollen from one plant to another.
- Collect pea seeds from the pollinated plants.
- Grow the seeds in favourable conditions.
- Observe the characteristics of the offspring.
- Cross-breed the offspring to observe future generations.
19.In a cross between two heterozygous parents (Hh × Hh), 8000 offspring are produced. How many offspring are expected to have the genotype hh?
Medium
- A2000
- B4000
- C6000
- D8000
20.A man with normal colour vision and a woman who is a carrier for colour-blindness have a child. What is the probability that the child will be colour-blind?
Medium
- A0%
- B25%
- C50%
- D100%
21.Which of the following statements about the inheritance of ABO blood groups are correct? (select all that apply)
Medium
- AI^A and I^B are codominant alleles
- BThe i allele is recessive to both I^A and I^B
- CA person with genotype I^A i has blood group A
- DA person with genotype I^A I^B has blood group O
- EThere are only two alleles for the ABO blood group gene
22.In a population, the distribution of height is plotted on a graph. Which type of variation does this represent?
Medium
- AContinuous variation
- BDiscontinuous variation
- CDiscrete variation
- DSex-linked variation
23.In humans, the sex of a child is determined by the chromosome carried by the sperm cell.
Easy
True or false?
24.In Mendel's experiments, why did he transfer pollen from one pea plant to the reproductive parts of another?
Medium
- ATo eliminate uncertainty about which pollen fertilised each plant
- BTo increase the rate of mutation in the offspring
- CTo ensure self-pollination occurred
- DTo produce genetically identical offspring
25.In a genetic cross, which pair of letters is the best choice for denoting dominant and recessive alleles when writing by hand?
Medium
- ACc
- BOo
- CHh
- DVv
26.In a Punnett grid for a cross between two heterozygous parents (Hh × Hh), what is the expected genotype ratio of the offspring?
Medium
- A1 HH : 2 Hh : 1 hh
- B3 HH : 1 hh
- C1 Hh : 1 hh
- D2 HH : 1 Hh : 1 hh
27.Which of the following statements about a Punnett grid are correct? (select all that apply)
Hard
- AIt predicts the probability of offspring genotypes.
- BIt shows the actual genotypes of all offspring produced.
- CIt can predict the numbers of offspring with a certain phenotype.
- DIt requires knowledge of the parental genotypes.
- EIt is used to determine the sex of the parents.
28.In a Punnett grid, gamete genotypes are placed inside the boxes to represent the possible offspring genotypes.
Easy
True or false?
29.Match each term related to genetic crosses with its correct description.
Medium
- P generation
- F1 generation
- F2 generation
- Punnett grid
- The first filial generation, offspring of the P generation
- The parental generation of pure-breeding parents
- A diagram used to predict offspring genotypes
- The second filial generation, offspring of the F1 cross
30.Place the steps of a monohybrid cross using a Punnett grid in the correct order.
Medium
- Write down the parental phenotypes and genotypes.
- Write down all possible gamete genotypes for each parent.
- Place each parental genotype on one axis of a 2x2 Punnett grid.
- Combine the gametes in each box to give offspring genotypes.
- List the phenotype and genotype ratios for the offspring.
31.A horticulturist wants to maintain a desirable trait in a plant. Which breeding method should be used to ensure the trait is passed on to the next generation?
Medium
- ASelf-pollination of the favoured plant
- BCross-pollination with a different plant
- CAllowing natural pollination by insects
- DGrowing the plant from cuttings
32.Which of the following are advantages of using artificial pollination in plant breeding? (select all that apply)
Hard
- AIt allows control over which plants are crossed.
- BIt increases genetic diversity in the offspring.
- CIt can be used to combine desirable traits from two parent plants.
- DIt ensures that all offspring are genetically identical.
- EIt eliminates the need for gametes.
33.In a monohybrid cross between two heterozygous parents, the expected phenotype ratio is 3:1 if one allele is completely dominant.
Medium
True or false?
34.Place the steps of Mendel's pea plant experiment in the correct order.
Hard
- Mendel transferred pollen from one pea plant to another.
- He collected the pea seeds from the cross-pollinated plants.
- He grew the seeds in favourable conditions to observe characteristics.
- He cross-bred the offspring to observe future generations.
35.What is the name of the process in which the nuclei of two gametes fuse to form a zygote?
Easy
- AFertilisation
- BMeiosis
- CPollination
- DMitosis
36.A zygote is a diploid cell containing two alleles of each gene.
Easy
True or false?
37.Which of the following describes the number of chromosomes in a human gamete?
Easy
- A23 single chromosomes
- B46 single chromosomes
- C23 pairs of chromosomes
- D46 pairs of chromosomes
38.Which of the following is the best definition of a gene?
Medium
- AA short length of DNA that codes for a particular characteristic by coding for a specific protein
- BA variation of a gene that codes for a different version of a characteristic
- CThe combination of alleles inherited from both parents
- DA structure made of DNA and protein that carries many genes
39.Which of the following statements about dominant and recessive alleles are correct? (select all that apply)
Medium
- AA dominant allele only needs to be inherited from one parent to be expressed in the phenotype
- BA recessive allele needs to be inherited from both parents to be expressed in the phenotype
- CIf only one recessive allele is present, it will remain hidden and the dominant characteristic will show
- DA recessive allele is always expressed in the phenotype when present
- EA dominant allele must be inherited from both parents to be expressed
40.Codominance occurs when both alleles are expressed to an equal extent in the phenotype, such as in speckled chickens.
Easy
True or false?
41.In a cross between two heterozygous parents (Hh × Hh), what percentage of offspring are expected to have the genotype hh?
Medium
42.Complete the sentence about sex determination in humans.
Easy
Females have the sex chromosomes ____, while males have the sex chromosomes ____.
43.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a sperm cell of this species?
Medium
44.A couple has a 50% chance of having a girl. If they have 4 children, what is the expected number of girls?
Medium
45.Complete the sentence about the inheritance of sex in humans.
Medium
A human female has the sex chromosomes ____, while a human male has the sex chromosomes ____.
46.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a fur-producing cell of this species?
Medium
47.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a muscle cell of this species?
Medium
48.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a zygote of this species?
Medium
49.A cross is performed between two heterozygous parents, Hh × Hh. If 8000 offspring are produced, how many would be expected to have the genotype hh? Show your working.
Medium
50.A cross is performed between two heterozygous parents, Hh × Hh. If 8000 offspring are produced, how many would be expected to show the dominant phenotype? Show your working.
Medium
51.A couple is expecting a child. Using a Punnett square for sex determination (X and Y chromosomes), calculate the percentage chance that the baby will be a girl. Show your working.
Medium
52.A man with normal colour vision (X^B Y) has a partner who is a carrier of the allele for colour-blindness (X^B X^b). What is the probability, as a percentage, that any child they have will be colour-blind? Show your working.
Medium
53.Define the term 'haploid'.
Easy
54.List two advantages of cells containing two sets of chromosomes.
Medium
55.Define the term genotype.
Medium
56.Describe the factors that can cause variation in hair colour.
Medium
57.State the type of organism that Mendel used in his studies.
Easy
58.In a cross between two heterozygous parents (Hh × Hh), what percentage of offspring are expected to have the genotype hh?
Medium
- A25 %
- B22 %
- C28 %
- D31 %
59.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a sperm cell of this species?
Medium
- A38 chromosomes
- B29 chromosomes
- C32 chromosomes
- D35 chromosomes
60.A couple has a 50% chance of having a girl. If they have 4 children, what is the expected number of girls?
Medium
- A3 girls
- B2 girls
- C1 girls
- D4 girls
61.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a fur-producing cell of this species?
Medium
- A70 chromosomes
- B58 chromosomes
- C64 chromosomes
- D76 chromosomes
62.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a muscle cell of this species?
Medium
- A70 chromosomes
- B58 chromosomes
- C76 chromosomes
- D64 chromosomes
63.A species of rodent has a diploid number of 64 chromosomes. How many chromosomes are present in a zygote of this species?
Medium
- A76 chromosomes
- B70 chromosomes
- C64 chromosomes
- D58 chromosomes
64.A cross is performed between two heterozygous parents, Hh × Hh. If 8000 offspring are produced, how many would be expected to have the genotype hh? Show your working.
Medium
- A1800 offspring
- B2400 offspring
- C2200 offspring
- D2000 offspring
65.A cross is performed between two heterozygous parents, Hh × Hh. If 8000 offspring are produced, how many would be expected to show the dominant phenotype? Show your working.
Medium
- A6600 offspring
- B5400 offspring
- C6000 offspring
- D7200 offspring
66.A couple is expecting a child. Using a Punnett square for sex determination (X and Y chromosomes), calculate the percentage chance that the baby will be a girl. Show your working.
Medium
- A45 %
- B55 %
- C50 %
- D60 %
67.A man with normal colour vision (X^B Y) has a partner who is a carrier of the allele for colour-blindness (X^B X^b). What is the probability, as a percentage, that any child they have will be colour-blind? Show your working.
Medium
- A28 %
- B22 %
- C31 %
- D25 %

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