Posted: November 3rd, 2015

Chromosomal linkage and maps

Chromosomal linkage and maps

1. (4 marks) In a plant, leaf color and leaf shape are controlled by two linkedgenes. Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed, and a recessive mutation in this gene causes them to be smooth. The following crosses were performed:

Cross 1: pure breeding white, smooth X pure breeding wild type gives an F1 where allthe progeny are red and pointed

Cross 2: red, pointed F1 X pure breeding white, smooth (test cross) gives an F2:
40 white, smooth
36 red, pointed
10 white, pointed
14 red, smooth

What is the recombination frequency between the gene for color and for shape?

2. (10 marks) In pea plants, the peas can be either rough or smooth, and either soft or hard. You do some crosses and obtain the following results.
cross 1: purebred rough, soft pea X purebred smooth, hard pea gives F1: all rough, hard

cross 2: rough, hard F1 X smooth, soft (note: test cross, NOT self cross)
gives F2: 115 rough soft, 110 smooth hard, 8 rough hard and 12 smooth soft

a) using letters of your own choosing, state the genotypes of the parents, the F1, and each phenotypic class in the F2.
b) which F2 progeny inherited PT (parental type) gametes from the F1 parent, and which inherited NPT (nonparental type)?
c) are the genes for roughness and softness linked? If so, what is the recombination frequency (distance in map units) between the genes?

3. (8 marks) Brown seed pods (B) in a plant species is is dominant to green (b), and elongated pods (E) is dominant over squished (e).
(a) A fully heterozygous plant has the dominant alleles linked in trans (i.e.
dominant alleles not on the same homologue) at a map distance of 20 cM. What
will be the genotypes of gametes produced by this plant, and in what frequencies
(or percentages)?
(b) If this plant is self-pollinated, what progeny phenotypes will you expect to see,
and in what frequencies? Use a Punnett square to illustrate your answer.

4. (6 marks) In corn…
Colored kernels (C) is dominant over colorless (c)
Plump kernels (S) is dominant over shrunken (s)
Starchy kernels (W) is dominant over waxy (w).
A trihybrid (Cc Ss Ww) plant is testcrossed and the following progeny are obtained:

2708 Colorless, plump, waxy
2538 Colored, shrunken, starchy
626 Colorless, plump, starchy
601 Colored, shrunken, waxy
116 Colorless, shrunken, starchy
113 Colored, plump, waxy
4Colored, plump, starchy
4 Colorless, shrunken, waxy
Determine linkage (including map distance) for the genes and gene order for this cross.

5. (8 marks) Alkaptonuria is an extremely rare disease. The gene for Alkaptonuria(ALK) has recently been shown to lie on human chromosome 9 and to be linked to the gene encoding the ABO blood group, with a recombination frequency of 11% between the loci.
A pedigree of a family with the disease is shown below, with affected individuals indicated in black. In addition, the blood type of family members is given.
The two alleles at the ALK locus will be denoted ALK+ and ALK-. The three alleles at the ABO blood group locus will be denoted IA, IB (which are co-dominant) and i (which is recessive to IA and IB).
(a) What is the genotype of individual 1 at the ALK and ABO loci?
(b) What is the genotype of individual 2 at the ALK and ABO loci?
(c) What is the genotype of individual 3 at the ALK and ABO loci? Which alleles of each gene are carried on the chromosome he inherited from his father and which alleles are carried on the chromosome he inherited from his mother?
(d) Individuals 3 and 4 are expecting their fifth child. A physician draws a prenatal blood sample and determines that the child has blood type B. What is the probability that the child will have alkaptonuria? Explain your answer.

6.(4 marks) An individual has the genotype Ab/aB. Gene loci (A) and (B) are 15 cM
apart. Indicate all the possible gametes this individual can produce, and
the proportions of expected progeny genotypes if a testcross is
performed on this individual.

7.(6 marks) In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1are then crossed with banded, yellow snails (a testcross).
a. What will be the results of the testcross if the loci that control banding and color
are linked with no crossing over?
b. What will be the results of the testcross if the loci assort independently?
c. What will be the results of the testcross if the loci are linked and 20 map units
apart?

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