D3.2 Inheritance

What is a difference between two alleles of a gene?

A. Their positions on homologous chromosomes

B. Their amino acid sequence

C. The characteristic they influence

D. Their base sequence

Answer: D

A. Incorrect. Both alleles of a gene occupy the same locus on homologous chromosomes, so their position is the same, not different.

B. Incorrect. The amino acid sequence may differ only if the alleles code for different versions of the protein, but the defining difference between alleles is at the DNA level.

C. Incorrect. Both alleles influence the same characteristic, although they may produce different variants of the trait.

D. Correct. Two alleles of a gene differ in their DNA base sequence, which can lead to differences in the protein produced or in gene expression.

A couple have four children whose blood groups are A, B and AB. What is the likely combination of the parents’ genotypes?

A.  I^Ai and I^Bi

B.  I^Ai and I^BI^B

C.  I^AI^B and ii

D.  I^AI^A and I^BI^B

Answer: A

A. Correct. This combination can produce: I^Ai (A), I^Bi (B), I^AI^B (AB) and ii (O). 

B. Incorrect. This combination can produce: I^Bi (B) and I^AI^B (AB).

C. Incorrect. This combination can produce I^Ai (A) and I^Bi (B).

D. Incorrect. This combination can only produce I^AIB (AB).

Black, short-haired guinea pigs, heterozygous for both characteristics, were crossed. They produced offspring with the phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1. A different cross produced offspring with phenotypes in the ratio 1:1:1:1. What were the genotypes of the parents in the second cross?

A.  BbSs × BbSs

B.  BBSs × BbSS

C.  BbSs × bbss

D.  bbSS × BBss

Answer: C

A. Incorrect. The phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1, means that for the color: black is dominant (B) and white is recessive (b), and for the length: short-haired is dominant (S) and long-haired is recessive (s). The BbSs x BbSs cross will produce the phenotypes black short-haired, black long-haired, white short-haired and white long-haired with the ratio of 9:3:3:1.

B. Incorrect. The phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1, means that for the color: black is dominant (B) and white is recessive (b), and for the length: short-haired is dominant (S) and long-haired is recessive (s). The BBSs x BbSS cross will produce only the phenotype black short-haired.

C. Correct. The phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1, means that for the color: black is dominant (B) and white is recessive (b), and for the length: short-haired is dominant (S) and long-haired is recessive (s). The BbSs x bbss cross will produce the phenotypes black short-haired, black long-haired, white short-haired and white long-haired with the ratio of 1:1:1:1. 

D. Incorrect. The phenotypes black short-haired, black long-haired, white short-haired and white long-haired in the ratio 9:3:3:1, means that for the color: black is dominant (B) and white is recessive (b), and for the length: short-haired is dominant (S) and long-haired is recessive (s). The bbSS x BBss cross will only produce the phenotype black short-haired.

Which genotype would be normally found in a gamete?

A. Rr

B. RS

C. rStt

D. TUt

Answer: B

A. Incorrect. Gametes are haploid, so they carry only one allele of each gene. Rr has two alleles of the same gene, which is diploid, not a gamete.

B. Correct. A gamete carries one allele for each gene. RS represents one allele of gene R and one allele of gene S, which is a valid haploid combination.

C. Incorrect. Gametes are haploid, so they carry only one allele of each gene. rStt has two alleles of the same gene, which is diploid, not a gamete.

D. Incorrect. Gametes are haploid, so they carry only one allele of each gene. TUt has two alleles of the same gene, which is diploid, not a gamete.

In the fruit fly Drosophila, the alleles for red eyes and brown bodies are dominant to the alleles for white eyes and yellow bodies. Which cross would be suitable to determine whether the genes are linked?

A. Heterozygous, red-eyed flies crossed with yellow-bodied flies

B. White-eyed flies crossed with homozygous, brown-bodied flies

C. Homozygous red-eyed, brown-bodied flies crossed with white-eyed, yellow-bodied flies

D. Heterozygous red-eyed, brown-bodied flies crossed with white-eyed, yellow-bodied flies

Answer: D

A. Incorrect. This cross only varies one trait in one parent and does not allow analysis of whether two genes are inherited together, so it cannot test for linkage.

B. Incorrect. This cross only varies one trait in one parent and does not allow analysis of whether two genes are inherited together, so it cannot test for linkage.

C. Incorrect. This cross only produces heterozygous F1 (RrBr), which cannot show linkage on its own. To detect linkage, the heterozygous F1 must be crossed with a homozygous recessive parent to observe parental and recombinant offspring.

D.Correct. This is a cross test. One parent is heterozygous for both traits, and the other is homozygous recessive for both traits, allowing observation of recombinant and parental phenotypes to determine if the two genes are linked.

Outline how experiments into inheritance can be performed using Drosophila (fruit flies) and what has been discovered by carrying out such experiments. [4]

Four of the following: 

a. cross males/fruit flies of one strain/with a certain trait/characteristic/phenotype with females/fruit flies of another / fruit flies with different characteristics crossed

b. study/analyze the progeny/offspring of the cross

c. dihybrid crosses/the inheritance of two genes/genetic traits is investigated together/at the same time

d. Morgan discovered sex linkage/can be used to study sex-linkage ratios are different for males and females/males XY and females XX;

e. Morgan’s discovery of non-Mendelian ratios

f. autosomal linkage/groups of genes that are on the same chromosome/ genes arranged in a linear sequence along the chromosome / gene mapping

g. Drosophila/fruit flies useful in inheritance experiments due to short life cycle/many generations can be studied in a short span / can be easily grown in the lab/ large number of offspring produced

Sample answer:

Experiments into inheritance using Drosophila involve crossing males of one strain with a particular trait with females of another strain having different traits [1]. The progeny of the cross are studied and analyzed [1]. Dihybrid crosses can be used to investigate the inheritance of two genes simultaneously [1]. Morgan discovered sex linkage through these experiments, showing that genes on the X chromosome produce different ratios in males (XY) and females (XX) [1]. He also observed non-Mendelian ratios and discovered autosomal linkage [1], where genes on the same chromosome are inherited together and arranged linearly, allowing gene mapping [1]. Drosophila are especially useful in inheritance studies because they have a short life cycle, produce many offspring, and can be easily maintained in the laboratory [1].

Explain the reasons for variation in human height. [7]

Seven of the following:

a. environment affects height

b. nutrition/malnutrition affects growth rate / other example of environmental factor affecting height

c. genes/alleles affect height / height is partly heritable

d. polygenic / many genes influence height

e. continuous variation

f. normal/bell-shaped distribution of height

g. some alleles of these genes increase height and some reduce it

h. many possible combinations of alleles of these genes

i. specific gene mutations/alleles cause dwarfism/extreme height

j. meiosis generates variation in height

k. mutations generate variation in height

l. males tend to be/are on average taller than females

m. loss of height during aging

Sample answer:

Variation in human height is influenced by environmental factors [1], such as nutrition and malnutrition, which affect growth rate [1]. Genetic factors also affect height, and it is partly heritable [1]. Height is a polygenic trait controlled by many genes [1], with some alleles increasing height and others reducing it [1]. There are many possible combinations of these alleles, leading to continuous variation in height [1]. Specific gene mutations can cause extreme height differences, such as dwarfism [1]. Additional sources of variation include meiosis and mutations [1]. Other influences include sex, with males on average taller than females, and loss of height during aging [1].

Distinguish between continuous and discrete variation, using examples. [4]

Four of the following:

Sample answer: 

Continuous variation shows no distinct categories and many possible phenotypes, while discrete variation shows distinct, non-overlapping classes with few possible phenotypes [1]. Continuous variation is controlled by multiple genes (polygenic), whereas discrete variation is controlled by one or few genes [1]. Continuous traits are influenced by environmental factors, while discrete traits are generally not affected by the environment [1]. Examples of continuous variation include height, weight, skin color, and intelligence, whereas examples of discrete variation include blood groups and number of eggs produced [1].

Explain gene linkage and its effects on inheritance. [2]

Two of the following: 

a. located on the same chromosome

b. genes/gene loci close together (on the same chromosome)

c. do not follow (the law of) independent assortment

d. more chance of recombination if genes are further apart

e. inherited together unless crossing over/recombination occurs

f. ratios of offspring in dihybrid crosses are different from expected/non-Mendelian OR more offspring with parental phenotype combinations than expected

Sample answer:

Gene linkage occurs when two genes are located on the same chromosome [1]. Genes that are close together on a chromosome are more likely to be inherited together [1]. Linked genes do not follow the law of independent assortment [1]. The further apart the genes are, the higher the chance of recombination occurring between them [1]. Linked genes are usually inherited together unless crossing over/recombination separates them [1]. As a result, the ratios of offspring in dihybrid crosses can differ from expected Mendelian ratios [1].

Outline the inheritance of blood types in the ABO blood system in humans. [4]

Four of the following: 

a. i and I^A and I^B are alleles of the blood group gene

b. I^A is dominant and i is recessive / I^B is dominant and i is recessive 

c. Group O only with ii and Group A with I^Ai or I^AI^A / Group B with I^Bi or I^BI^B

d. I^A and I^B are co-dominant so Group AB with I^AI^B 

e. one allele/copy of the gene inherited from each parent/Punnett square showing this 

Sample answer:

The ABO blood system in humans is determined by three alleles: i, I^A, and I^B [1]. I^A and I^B are dominant over i, which is recessive [1]. Blood group O has genotype ii, group A has I^Ai or I^AI^A, and group B has I^Bi or I^BI^B [1]. I^A and I^B are co-dominant, so an individual with I^AI^B has blood group AB [1]. Each person inherits one allele from each parent [1].