47. The following pedigree shows the inheritance of red hair in a family in Scot

Question

47. The following pedigree shows the inheritance of red hair in a family in Scotland. Red hair is caused by homozygosity for a recessive allele of a gene called MCIR. Although worldwide red hair is the rarest of human hair colors, red hair is not uncommon in Scotland. In fact, 40% of Scots without red hair are nonetheless carriers of the red hair allele. 9 10 2 a. Why does red hair show a horizontal inheritance pattern in this particular pedigree even though the trait is caused by a recessive allele? b. Assuming that individual III-2 has a child with the Scottish woman shown (III-1) who is not a close relative, what is the probability that this child (IV-1) will have red hair? c. What is the probability that the child of first cous- ins III-9 and III-10 (IV-2), will have red hair?

Explanation / Answer

a. Since the red hair inheritance is caused due to presence of homozygous recessive allele. So the females who will have XX recessive will be affected by the trait, and the ones which will have heterozygous alleleic condition, will not be affected but carrier of that particular trait.Since this red hair inheritance is X- linked, so the males who will inherit One X chromosome with the red hair allele, will be having red hair colour.

Genotype of Males (Normal) = XY

(Red hair)= XRY

Genotype of females (Normal)= XX

                                 (Carrier) = XRX

                            (Red Hair) = XRXR

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B.

b. Probability of IV-1 having red hair

Since III-2 is a male with red hair, genotype III-2 = XRY

There may be two cases for III-1. She may be either normal (XX) or Carrier XRX

Case 1. If She is a carrier

XR

Y

XR

XRXR

(Red hair female)

XRY

(Red hair male)

X

XRX

(Normal carrier female)

XY

(Normal male)

So the Probability of IV-1 with red hair = Red hair female : ¼ or 25%

                                                                        Red hair male : ¼ or 25%

                                                                        Normal carrier female :: ¼ or 25%

                                                                        Normal male : ¼ or 25%

So, probability of a red haired child = ½ or 50%

            Case -2 : If She is normal                                          

XR

Y

X

XRX

(Normal carrier female)

XY

(Normal male)

X

XRX

(Normal carrier female)

XY

(Normal male)

Probability of Red hair child = 0

There will be 50% carrier female and 50 % normal male.

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c. Probability of IV-2 with Red hair:

It is shown in the pedigree tat III-10 is a normal male, but since III-9 is a child of red hair female, III-9 may be either carrier or normal. Here also there will be two cases.

Case 1: If III-9 is a carrier

X

Y

XR

XRX

(Normal carrier female)

XRY

(Red hair male)

X

XX

(Normal female)

XY

(Normal male)

Probability of Red hair child = ¼ or 25%. Rest all the children do not show thw red hair phenotype.

Case -2: If III-9 is a normal XX

X

Y

X

XX

(Normal female)

XY

(Normal male)

X

XX

(Normal female)

XY

(Normal male)

Probability of Red hair child =0.

All the progenies will be normal.

XR

Y

XR

XRXR

(Red hair female)

XRY

(Red hair male)

X

XRX

(Normal carrier female)

XY

(Normal male)

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