16. You are studying a species of fruit fly, Drosophila mitensis, and you find a fly that has cut wings as seen here (use “w+” as wild type allele and “cut” as cut wings allele):
(a) Female fruit flies with normal wings were crossed to males with malformed "cut" wings. The F1 progeny were:
1/4 normal females
1/4 cut-wing females
1/4 normal males
1/4 cut-wing males
State if each of the following modes of inheritance can explain these results (1 mark each), and if so, what the genotypes of the parents must be (2 marks each). Use Punnett squares to determine the outcomes. (12 marks)
(i) Cut wing is autosomal recessive ________
(ii) Cut wing is autosomal dominant ________
(iii) Cut wing is X-linked recessive _________
(iv) Cut wing is X-linked dominant _________
(i) Yes, the genotypes of the parents must be: Female: w+w+; Male: w+cut.
(ii) No.
(iii) Yes, the genotypes of the parents must be: Female: w+w+; Male: w+Xcut.
(iv) No.
(i) Cut wing is autosomal recessive - YES
To explain the results, the genotypes of the parents must be:
Female (Normal Wings): w+ w+
Male (Cut Wings): cut cut
Punnett Square:
w+ cut
w+ w+ w+ w+ cut
w+ w+ w+ w+ cut
Offspring:
1/4 Normal females: w+ w+
1/4 Cut-wing females: w+ cut
1/4 Normal males: w+ w+
1/4 Cut-wing males: w+ cut
(ii) Cut wing is autosomal dominant - NO
If cut wing was autosomal dominant, we would expect all the offspring to have cut wings since even one copy of the allele would result in the phenotype. However, we observe normal females and normal males in the F1 progeny.
(iii) Cut wing is X-linked recessive - NO
If cut wing was X-linked recessive, we would expect more males to have cut wings since they inherit their X chromosome from their mothers. However, we observe an equal number of cut-wing females and cut-wing males in the F1 progeny.
(iv) Cut wing is X-linked dominant - NO
If cut wing was X-linked dominant, we would expect all the offspring to have cut wings since even one copy of the allele would result in the phenotype. However, we observe normal females and normal males in the F1 progeny.
(i) Cut wing is autosomal recessive
Explanation: This mode of inheritance can explain the results. In this case, the cut wing trait is caused by a recessive allele. We can represent the genotypes of the parents using Punnett squares:
Female parent: w+/w+ (normal wings)
Male parent: cut/w+ (cut wing)
Punnett square:
| w+ | cut
------------------------------
w+ | w+/w+ | w+/cut
------------------------------
w+ | w+/w+ | w+/cut
There is a 1/4 chance of producing offspring with the cut wing phenotype, which matches the observed F1 progeny results.
(ii) Cut wing is autosomal dominant
Explanation: This mode of inheritance cannot explain the results, as the F1 progeny include individuals with the normal wing phenotype.
(iii) Cut wing is X-linked recessive
Explanation: This mode of inheritance cannot explain the results, as the F1 progeny include both males and females with the cut wing phenotype. X-linked recessive traits are typically more common in males.
(iv) Cut wing is X-linked dominant
Explanation: This mode of inheritance cannot explain the results, as the F1 progeny include individuals with the normal wing phenotype. X-linked dominant traits would be present in all individuals inheriting the allele.
To determine whether each mode of inheritance can explain the given results, we need to consider the expected outcomes of each cross and compare them with the observed F1 progeny.
(i) Cut wing is autosomal recessive:
In autosomal recessive inheritance, the trait will only be expressed (phenotype) when an individual is homozygous for the recessive allele. The wild type allele, "w+", represents the dominant allele, and the "cut" allele represents the recessive allele.
To test this mode of inheritance, we would expect that a female with normal wings (genotype: ww+, where w represents the wild type allele) is crossed with a male with malformed "cut" wings (genotype: wc, where c represents the cut allele).
The Punnett square for this cross would be:
| w w+
----------------
w c | wc wc+
w c | wc wc+
The expected F1 progeny would be 100% heterozygous for the cut allele (genotype: wc+), which would result in a 50% occurrence of normal females and a 50% occurrence of cut-wing females.
Since the observed F1 progeny shows 1/4 normal females and 1/4 cut-wing females, this mode of inheritance (autosomal recessive) cannot explain the results.
(ii) Cut wing is autosomal dominant:
In autosomal dominant inheritance, the trait will be expressed (phenotype) when an individual carries at least one copy of the dominant allele. The wild type allele, "w+", represents the dominant allele, and the "cut" allele represents the recessive allele.
To test this mode of inheritance, we would expect that a female with normal wings (genotype: ww+ or w+w+) is crossed with a male with malformed "cut" wings (genotype: wc, where c represents the cut allele).
The Punnett square for this cross would be:
| w w+
----------------
w c | wc wc+
w c | wc wc+
The expected F1 progeny would be 100% heterozygous for the cut allele (genotype: wc+), which would result in a 50% occurrence of normal females and a 50% occurrence of cut-wing females.
Since the observed F1 progeny shows 1/4 normal females and 1/4 cut-wing females, this mode of inheritance (autosomal dominant) cannot explain the results.
(iii) Cut wing is X-linked recessive:
In X-linked recessive inheritance, the trait will be expressed (phenotype) when an individual carries two copies of the recessive allele on the X chromosome. Since fruit flies have two X chromosomes in females and one X chromosome in males, the inheritance pattern can differ between the two sexes.
To test this mode of inheritance, we would expect that a female with normal wings (genotype: w+w+) is crossed with a male with malformed "cut" wings (genotype: wc, where c represents the cut allele).
The Punnett square for this cross would be:
| w+ w
----------------
w c | w+ wc
w + | w+ w+
The expected F1 progeny would be 50% normal females (genotype: w+w+) and 50% carriers (genotype: w+wc) that appear normal but carry the cut allele.
Since the observed F1 progeny shows 1/4 normal females and 1/4 cut-wing females, this mode of inheritance (X-linked recessive) can partially explain the results. However, it does not explain the occurrence of normal males.
(iv) Cut wing is X-linked dominant:
In X-linked dominant inheritance, the trait will be expressed (phenotype) when an individual carries at least one copy of the dominant allele on the X chromosome. Like X-linked recessive inheritance, the inheritance pattern can differ between females and males.
To test this mode of inheritance, we would expect that a female with normal wings (genotype: w+w+) is crossed with a male with malformed "cut" wings (genotype: wc, where c represents the cut allele).
The Punnett square for this cross would be:
| w+ w
----------------
w c | wc wc
w + | w+ w+
The expected F1 progeny would be 50% normal females (genotype: w+w+) and 50% cut-wing females (genotype: wcwc).
Since the observed F1 progeny shows 1/4 normal females and 1/4 cut-wing females, this mode of inheritance (X-linked dominant) can fully explain the results.
In conclusion:
- Cut wing is not autosomal recessive, autosomal dominant, or X-linked recessive.
- Cut wing is X-linked dominant, and the genotypes of the parents are as follows:
- The female parent has normal wings (genotype: w+w+).
- The male parent has malformed "cut" wings (genotype: wc).