Inheritance (A Level Only)

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Key Terms in Genetics

A gene is a DNA sequence that codes for a polypeptide or protein, located at a specific locus on a chromosome.
Different versions of a gene are called alleles; they have slightly different base sequences but occupy the same locus.
The genotype is the alleles an organism possesses; the phenotype is the observable characteristics resulting from the genotype and the environment.
Individuals can be homozygous (two identical alleles) or heterozygous (two different alleles) for a gene.
A dominant allele is always expressed in the phenotype, even in heterozygotes; a recessive allele is only expressed when no dominant allele is present (i.e. in homozygotes).
Codominance occurs when both alleles in a heterozygote are fully expressed, so features of both alleles are seen in the phenotype.
In codominance, genotype notation uses a capital letter for the gene and superscript letters for alleles, e.g. Iᴬ, Iᴮ, Iᴼ for human blood groups.

Monohybrid Crosses

Monohybrid inheritance tracks the inheritance of one gene with two alleles.
During gamete formation, each allele from a homologous pair has an equal chance of being passed on, so the zygote has an equal probability of inheriting either parental allele.
Genetic diagrams (especially Punnett squares) are used to predict offspring genotypes and phenotypes.
A cross between two heterozygotes (e.g. Bb × Bb) gives a predicted phenotypic ratio of 3 dominant : 1 recessive and a genotypic ratio of 1 BB : 2 Bb : 1 bb.
Predictions are based on probability; observed ratios may differ due to random fertilisation of gametes.
For codominant alleles, there are more possible phenotypes, so predicted ratios differ from simple dominant/recessive crosses.
In sex-linked inheritance, males (XY) have one copy of X-linked genes, so are more likely to express recessive traits; females (XX) can be unaffected, carriers, or affected.

Sex Linkage and Haemophilia

Sex-linked genes are located on the X chromosome (rarely on the Y chromosome).
Males have one X and one Y chromosome, so they have only one allele for X-linked genes; they cannot be carriers of recessive X-linked traits.
Females have two X chromosomes, so they can be homozygous dominant (unaffected), heterozygous (carriers), or homozygous recessive (affected).
Haemophilia is a recessive X-linked disorder caused by a gene coding for Factor VIII, a protein essential for blood clotting.
Males need only one copy of the recessive allele (XᵃY) to be affected; females must inherit two copies (XᵃXᵃ) to be affected.
A carrier female (XᴬXᵃ) crossed with a normal male (XᴬY) gives a 1:1:1:1 ratio of normal female : carrier female : normal male : affected male.
Males cannot pass X-linked traits to their sons because they only pass on the Y chromosome to sons.

Dihybrid Crosses and Linkage

Dihybrid crosses track the inheritance of two genes simultaneously, involving more genotype and phenotype combinations.
In genotype notation, write both alleles of one gene first, then both alleles of the second gene (e.g. YyGg); do not mix alleles between genes.
Dihybrid crosses assume independent assortment of genes during meiosis, giving a typical 9:3:3:1 phenotypic ratio when two heterozygotes are crossed.
Autosomal linkage occurs when two or more genes are on the same chromosome; they do not assort independently and tend to be inherited together, maintaining the parental allele combination.
Linked alleles are often written in brackets to show they are inherited together, e.g. (FG)(FG) instead of FFGG.
Linkage reduces variation and alters expected offspring ratios, so they deviate from standard dihybrid predictions.
Crossing over during meiosis can produce recombinant offspring, so not all offspring show the parental combinations.

Epistasis

Epistasis occurs when one gene affects the expression of another gene, involving two genes on different chromosomes influencing the same trait.
The interaction between alleles at different loci determines the resulting phenotype.
Epistasis must be considered when predicting phenotypic ratios in genetic crosses.
Example: in mice, the allele for black fur (B) is dominant to brown (b), but a homozygous recessive genotype (cc) at a second gene prevents pigment production, resulting in albino mice.
Example: in pigeons, the dominant allele F stops grey feather production even if the allele R for grey pigment is present, so only rrff birds have grey feathers.
Epistasis can produce modified phenotypic ratios that do not fit standard Mendelian predictions.

Test Crosses

A test cross determines the genotype of an individual showing a dominant phenotype by crossing it with a homozygous recessive individual.
The homozygous recessive individual has a known genotype (e.g. hh, rr, bb) and expresses the recessive phenotype.
For a monohybrid test cross: if no recessive offspring appear, the unknown is homozygous dominant; if at least one recessive offspring appears, the unknown is heterozygous.
For a dihybrid test cross: no recessive phenotypes for either gene means the unknown is homozygous dominant for both genes.
If at least one recessive phenotype appears for one gene only, the unknown is heterozygous for one gene and homozygous dominant for the other.
If at least one recessive phenotype appears for both genes, the unknown is heterozygous for both genes.

Investigating Genetic Ratios

Genetic diagrams predict offspring genotypes and phenotypes using known parental genotypes and meiosis, but actual ratios may differ due to random fertilisation.
Model organisms such as Fast Plant® (rapid-cycling Brassica rapa) and Drosophila (fruit flies) are used because they have short life cycles and easily identifiable traits controlled by single genes.
In Fast Plant®, stem colour is controlled by a single gene: A = dominant (anthocyanin present, purple stem), a = recessive (no anthocyanin, green stem).
A cross between two heterozygous Fast Plants (Aa × Aa) gives a predicted 3 purple : 1 green ratio.
In Drosophila, wing length is controlled by a single gene: L = dominant (long/wild-type wing), l = recessive (vestigial wing).
A cross between two heterozygous Drosophila (Ll × Ll) gives a predicted 3 wild-type : 1 vestigial ratio (≈75% : 25%).
In a dihybrid cross, a 9:3:3:1 ratio means four possible phenotypes with probabilities of 56.25%, 18.75%, 18.75% and 6.25%.

The Chi-squared Test

The chi-squared test determines whether there is a significant difference between observed and expected results in an experiment.
It is used when data is categorical, i.e. falls into distinct groups.
The chi-squared value is calculated using the formula χ² = Σ(O − E)² ÷ E, where O = observed and E = expected.
The degrees of freedom are calculated as the number of classes minus 1.
Biologists generally use a probability level of 0.05 (5%); the critical value is read from a table using this probability and the degrees of freedom.
If χ² ≥ critical value, there is a significant difference not due to chance; reject the null hypothesis.
If χ² < critical value, there is no significant difference; accept the null hypothesis as any difference is due to chance.
A significant difference may suggest an unaccounted factor such as linkage between genes.

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1.What is the definition of a gene?
Easy
- AA DNA sequence that codes for a polypeptide or protein
- BA length of DNA that codes for a carbohydrate
- CA protein that controls a characteristic
- DA section of RNA that codes for a protein
2.Which of the following best defines the term 'genotype'?
Easy
- AThe observable characteristics of an organism
- BThe alleles possessed by an organism
- CThe physical location of a gene on a chromosome
- DThe number of chromosomes in a cell
3.In genetics, what is a sex-linked characteristic?
Easy
- AA characteristic controlled by a gene located on the X chromosome
- BA characteristic controlled by a gene on an autosome
- CA characteristic that is only expressed in males
- DA characteristic controlled by multiple genes
4.Which of the following correctly describes codominance?
Medium
- ABoth alleles in a heterozygous genotype are fully expressed, so features of both are observed
- BOne allele is always expressed in the phenotype, masking the other
- CThe phenotype is intermediate between the two homozygous phenotypes
- DOnly one allele is present in the genotype
5.Which of the following is the correct definition of epistasis?
Medium
- AOne gene affects the expression of another gene
- BTwo genes on the same chromosome are inherited together
- CBoth alleles in a heterozygote are fully expressed
- DA gene located on the X chromosome
6.In a monohybrid cross between two heterozygous individuals (Aa x Aa), what is the expected phenotypic ratio of dominant to recessive offspring?
Medium
- A3:1
- B1:1
- C1:2:1
- D9:3:3:1
7.A test cross is performed to determine the genotype of an individual showing a dominant phenotype. If all offspring show the dominant phenotype, what is the genotype of the unknown individual?
Medium
- AHomozygous dominant
- BHeterozygous
- CHomozygous recessive
- DCannot be determined
8.In a dihybrid cross where both parents are heterozygous for two independently assorting genes, what is the expected phenotypic ratio?
Medium
- A9:3:3:1
- B3:1
- C1:1:1:1
- D1:2:1

Thêm câu hỏi

9.What does a chi-squared value greater than the critical value at p=0.05 indicate?
Hard
- AThere is a significant difference between observed and expected results, so the null hypothesis is rejected
- BThere is no significant difference, so the null hypothesis is accepted
- CThe results are due to chance
- DThe experiment must be repeated
10.Which of the following is an example of a sex-linked disorder?
Medium
- AHaemophilia
- BCystic fibrosis
- CSickle cell anaemia
- DHuntington's disease
11.In humans, blood type is determined by multiple alleles. Which genotype results in blood type AB?
Medium
- AIᴬIᴮ
- BIᴬIᴬ
- CIᴮIᴮ
- DIᴼIᴼ
12.What is the definition of a recessive allele?
Medium
- AAn allele only expressed in the phenotype if no dominant allele is present
- BAn allele always expressed in the phenotype
- CAn allele that is only found on the X chromosome
- DAn allele that codes for a recessive characteristic
13.Which of the following statements about autosomal linkage is correct?
Hard
- AGenes on the same chromosome are inherited together and do not assort independently
- BGenes on different chromosomes are inherited together
- CLinked genes always show a 9:3:3:1 ratio in a dihybrid cross
- DAutosomal linkage involves genes on the sex chromosomes
14.What is the expected genotypic ratio from a cross between two heterozygous individuals (Aa x Aa)?
Medium
- A1 AA : 2 Aa : 1 aa
- B3 AA : 1 aa
- C1 Aa : 1 aa
- D1 AA : 1 Aa : 1 aa
15.In a test cross, if the unknown individual is heterozygous, what ratio of phenotypes is expected in the offspring?
Medium
- A1:1 dominant to recessive
- B3:1 dominant to recessive
- CAll dominant
- DAll recessive
16.Which of the following are true for sex-linked genes? (Select all that apply.)
Hard
- AThey are located on the X chromosome
- BMales are more likely to express recessive sex-linked traits
- CFemales can be carriers of recessive sex-linked traits
- DMales can be carriers of recessive sex-linked traits
- EThey are always located on the Y chromosome
17.Which of the following statements about epistasis are correct? (Select all that apply.)
Hard
- AIt involves two genes on different chromosomes
- BOne gene affects the expression of another gene
- CIt can alter expected phenotypic ratios
- DIt is the same as codominance
- EIt only occurs in sex-linked genes
18.Which of the following are reasons why observed ratios may differ from expected ratios in genetic crosses? (Select all that apply.)
Hard
- ARandom fertilisation of gametes
- BSmall sample size
- CLinkage between genes
- DAll alleles are dominant
- EThe genes are on different chromosomes
19.A gene is a sequence of DNA that codes for a polypeptide or protein.
Easy
True or false?
20.A recessive allele is always expressed in the phenotype.
Easy
True or false?
21.In codominance, both alleles in a heterozygote are fully expressed.
Medium
True or false?
22.Autosomal linkage refers to genes located on the sex chromosomes.
Medium
True or false?
23.The chi-squared test is used to determine if there is a significant difference between observed and expected results.
Medium
True or false?
24.Match the genetic term with its correct definition.
Medium
- Genotype
- Phenotype
- Codominance
- Epistasis
- The alleles possessed by an organism
- The observable characteristics of an organism
- Both alleles in a heterozygote are fully expressed
- One gene affects the expression of another gene
25.Match the type of inheritance with its description.
Hard
- Monohybrid cross
- Dihybrid cross
- Test cross
- Sex-linked inheritance
- Tracks inheritance of one gene
- Tracks inheritance of two genes
- Determines genotype of an individual showing dominant phenotype
- Genes located on the X chromosome
26.Place the steps of the chi-squared test in the correct order.
Medium
- Calculate the difference between observed and expected results
- Square each difference
- Divide each squared difference by the expected value
- Sum the values to obtain the chi-squared value
- Compare to critical value
27.Order the stages of a monohybrid cross from parent to offspring.
Medium
- Parental genotypes
- Gamete formation
- Fertilisation
- Offspring genotypes
- Offspring phenotypes
28.Match each key term in genetics to its correct description.
Easy
- Gene
- Allele
- Locus
- Phenotype
- A DNA sequence that codes for a polypeptide or protein
- A different version of a gene with a slightly different base sequence
- The specific position of a gene on a chromosome
- The observable characteristics determined by genotype and environment
29.Which of the following statements correctly describe codominance? (Select all that apply.)
Medium
- ABoth alleles in a heterozygote are fully expressed
- BFeatures of both alleles are observed in the phenotype
- CThe heterozygote shows an intermediate phenotype
- DThe genotype notation uses a capital letter for the gene and superscript letters for the alleles
- EOne allele masks the expression of the other
30.Place the steps of a chi-squared test in the correct order.
Medium
- Obtain the expected and observed results
- Calculate the difference between each observed and expected value
- Square each difference
- Divide each squared difference by the expected value
- Add the resulting values to obtain the chi-squared value
31.Match each type of inheritance with its correct description.
Medium
- Codominance
- Sex linkage
- Autosomal linkage
- Epistasis
- Both alleles in a heterozygote are fully expressed
- Genes located on the X chromosome
- Genes on the same autosome inherited together
- One gene affects the expression of another gene
32.Identify the type of allele interaction shown by the tortoiseshell phenotype.
Medium
- ACodominance
- BComplete dominance
- CEpistasis
- DSex linkage
33.A female tortoiseshell cat (XGXB) is crossed with a black male cat (XBY). What percentage of their offspring are expected to be black male cats?
Medium
- A25%
- B50%
- C75%
- D0%
34.Use a genetic diagram to determine the ratio of phenotypes expected from this cross.
Medium
- AAll offspring long, black
- B1 long, black : 1 long, white
- C3 long, black : 1 long, white
- D1 long, black : 1 short, black
35.If the genes were located on different homologous pairs of chromosomes, what is the expected ratio of phenotypes in the offspring?
Medium
- A9 : 3 : 3 : 1
- B1 : 1 : 1 : 1
- C3 : 1
- D9 : 7
36.The observed numbers were: non-prickly round = 2175, prickly round = 79, non-prickly linear = 67, prickly linear = 679. Which statements explain why the observed ratios do not match the expected 9 : 3 : 3 : 1 ratio? (Select all that apply.)
Hard
- AThe genes are linked on the same chromosome
- BCrossing over during meiosis produces some recombinant offspring
- CThe genes assort independently
- DRandom fertilisation causes small deviations from expected ratios
- EThe alleles show codominance
37.What phenotypic ratio is expected in the offspring?
Hard
- A4 white : 3 yellow : 1 green
- B9 : 3 : 3 : 1
- C1 : 1 : 1 : 1
- D3 white : 1 yellow
38.What conclusion should be drawn from these results?
Medium
- AThere is no significant difference between observed and expected results; accept the null hypothesis
- BThere is a significant difference; reject the null hypothesis
- CThe results prove linkage between the genes
- DThe chi-squared value is too large to interpret
39.A chi-squared value greater than the critical value indicates a significant difference between observed and expected results.
Easy
True or false?
40.In a species of newt, tail length and scale colour are controlled by genes showing autosomal linkage. A newt heterozygous for normal tail and green scales (TG)(tg) is crossed with a newt with short tail and white scales (tg)(tg). What phenotypic ratio is expected in the offspring?
Hard
- A1 normal tail, green scales : 1 short tail, white scales
- B9 : 3 : 3 : 1
- C3 : 1
- D1 : 1 : 1 : 1
41.Complete the sentence.
Easy
A ____ is a DNA sequence that codes for a polypeptide or protein.
42.In a chi-squared test with 4 phenotypic classes, what are the degrees of freedom?
Medium
43.A dihybrid cross produces 160 offspring. If the expected ratio is 9:3:3:1, how many offspring are expected to show the dominant phenotype for both traits?
Hard
44.Name the term for the alleles an organism possesses.
Medium
45.What is the term for a gene that affects the expression of another gene?
Medium
46.Complete the sentence about sex-linked genes.
Easy
Sex-linked genes are located on the ____ chromosome, and males are more likely to express recessive sex-linked traits because they have only one ____ chromosome.
47.Presence of another gene at a locus on a different chromosome results in cats with completely black fur. Identify the term used to describe this type of interaction between alleles.
Medium
48.State the genotypes of the two parent guinea pigs.
Medium
Long, black parent: ____ ; Long, white parent: ____
49.Suggest why the ratios that Mendel expected were not always the same as the actual ratios that were observed.
Easy
50.Complete the table to show the corresponding phenotype or genotype.
Medium
Non-prickly stem, linear fruit = ____ ; PpRR = ____ ; ppRr = ____ ; Prickly stem, linear fruit = ____
51.Identify the type of gene interaction shown by the effect of gene A on the expression of gene B.
Medium
52.Calculate the expected number of offspring showing both dominant phenotypes.
Medium
53.Explain why a male can only inherit haemophilia from his mother.
Hard
54.Explain how the results in Table 1 support the theory that the allele for green seed pods is dominant and the allele for yellow seed pods is recessive.
Hard
55.State why it is not usually possible to see male tortoiseshell cats.
Easy
56.Explain how these results support the theory that the allele for green seed pods is dominant and the allele for yellow seed pods is recessive.
Hard
57.In a chi-squared test with 4 phenotypic classes, what are the degrees of freedom?
Medium
- A3
- B2
- C4
- D5
58.A dihybrid cross produces 160 offspring. If the expected ratio is 9:3:3:1, how many offspring are expected to show the dominant phenotype for both traits?
Hard
- A81
- B108
- C99
- D90
59.Calculate the expected number of offspring showing both dominant phenotypes.
Medium
- A72 offspring
- B79 offspring
- C86 offspring
- D65 offspring

Ghi chú ôn tập

## Key Terms in Genetics - **Gene**: DNA sequence coding for a polypeptide/protein; located at a specific **locus** on a chromosome. - **Allele**: different version of a gene; slightly different base sequence, same locus. - **Genotype**: the alleles an organism possesses. - **Phenotype**: observable characteristics; determined by genotype and environment. - **Homozygous**: same alleles; **heterozygous**: different alleles. - **Dominant**: always expressed (in both homo- and heterozygous). **Recessive**: only expressed when homozygous (no dominant allele present). - **Codominance**: both alleles in a heterozygote are fully expressed (e.g. IᴬIᴮ → blood group AB). ## Monohybrid Crosses - Track one gene with two alleles. Each allele from a homologous pair has an equal chance of being passed on. - F₁ (AA × aa) → all Aa, dominant phenotype. F₁ × F₁ (Aa × Aa) → **3 dominant : 1 recessive**; genotypes **1 AA : 2 Aa : 1 aa**. - Predictions are based on probability; actual ratios may differ due to random fertilisation. ## Sex Linkage - Sex-linked genes are on the X chromosome (rarely Y). Males (XY) have one copy → more likely to express recessive traits; cannot be carriers and don't pass X-linked traits to sons. - Haemophilia: recessive X-linked; F = normal Factor VIII, f = no Factor VIII. - XᴬXᴬ unaffected; XᴬXᵃ carrier; XᵃXᵃ affected; XᴬY unaffected; XᵃY affected. ## Dihybrid Crosses - Track two genes. Write both alleles of one gene first, then the second (e.g. YyGg). - Heterozygous × heterozygous → typical **9:3:3:1** ratio. - **Autosomal linkage**: genes on the same chromosome inherited together, maintaining parental combinations; ratios deviate from 9:3:3:1. Linked alleles written in brackets, e.g. (FG)(FG). - **Epistasis**: one gene affects expression of another (two genes on different chromosomes). ## Test Crosses - Cross unknown dominant-phenotype individual with homozygous recessive. - Monohybrid: no recessive offspring → homozygous dominant; ≥1 recessive → heterozygous. - Dihybrid: no recessive → homozygous both; recessive for one gene → heterozygous for one; recessive for both → heterozygous both. ## Investigating Genetic Ratios - Model organisms (Fast Plant®, Drosophila) have short life cycles and single-gene traits. - Fast Plant®: A = purple stem (anthocyanin), a = green stem. Aa × Aa → 3:1. - Drosophila: L = wild-type wing, l = vestigial wing. Ll × Ll → 3:1. ## Chi-squared Test - Tests whether difference between observed (O) and expected (E) is significant. - **χ² = ∑(O − E)² ÷ E** - **degrees of freedom = number of classes − 1** - Use probability level 0.05 (5%). - χ² ≥ critical value → significant difference, not due to chance → reject null hypothesis. - χ² < critical value → no significant difference, due to chance → accept null hypothesis.

Gene: A DNA sequence that codes for a polypeptide or protein; located at a specific locus on a chromosome.
Allele: Different versions of a gene with slightly different base sequences that occupy the same locus.
Genotype: The alleles an organism possesses.
Phenotype: The observable characteristics, determined by the genotype and the environment.
Homozygous: Having the same alleles for a gene (e.g. AA or aa).
Heterozygous: Having different alleles for a gene (e.g. Aa).
Dominant allele: An allele that is always expressed in the phenotype, in both heterozygous and homozygous individuals.
Recessive allele: An allele only expressed in the phenotype when no dominant allele is present (homozygous).
Codominance: When both alleles in a heterozygous genotype are fully expressed, so features of both are observed.
Sex linkage: Genes located on the X chromosome (rarely Y); males (XY) have one copy so are more likely to express recessive traits.
Autosomal linkage: Genes on the same autosome that do not assort independently during meiosis and tend to be inherited together.
Epistasis: When one gene affects the expression of another gene, involving two genes on different chromosomes influencing the same trait.
Test cross: Crossing an individual with a dominant phenotype with a homozygous recessive individual to determine its genotype.
Chi-squared test: A statistical test determining whether there is a significant difference between observed and expected results.
Degrees of freedom: Calculated as number of classes − 1; used with the critical value table in the chi-squared test.

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