Interesting Questions from Genetics

hi friends ... just check out this question

The horse(Equus caballus) has a diploid complement of 64 chromosomes including 36 acrocentric autosomes;  the ass (Equus asinus ) has 62 chromosomes including 22 acrocentric autosomes.

a) predict the number of chromosomes to be found in the hybrid offspring (mule) produced by matinga male ass (jack) to a female horse(mare).

b) Why are mules usually sterile ?

and the answers are
first bit is easy ... second one is interesting

a) now as we know the sperm(jack) will carry haploid(n) set of chromosomes i.e; n = 31 here and the same way the egg (mare) will have n= 32 (haploid again) ; and hence the resulting hybrid mule wud have diplod set of chromosomes (formed by the union of gametes) with 31+ 32 = 63 chromosomes ...

b) as  seen in the question the haploid set of chromosome in horse has 18 acrocentric chromosomes , which are so dissimilar from the haploid set in jack having only 11 acrocentric chromosomes . so different that , meiosis in the mule germ line cannot proceed beyond first prophase where synapsis of homologues occur.. thereby mules are incapable of producing viable gametes ...they r sterile...

PROBLEM 2:
The absence of legs in cattle ("amputated") has been attributed to a completely recessive lethal gene. A normal bull is mated with a normal cow and they produce an amputated calf (usually death at birth). The same parents are mated again.

a) what is the chance that of the next calf being amputated?

b) what is the chance of these parents having two calves, both of which are amputated?

c) Bull carrying the amputated allele (heterozygous) are mated to noncarrier cows. The F1 is allowed to mate at random to produce the F2. What genotypic ratio is expected in the adult F2?

d)Suppose that each F1 female in part (c) rears one viable calf (i.e; each of the cows that throws an amputated calf is allowed to remate to a carrier sire until she produces a viable offspring). What genotypic ratio is expected in the adult F2?

(For convenience assign (AA Aa aa) as gene symbols.where "a" is the lethal gene)

the answers are -
a)25%
b)1/16
c)9AA:6Aa (or 3AA:2Aa)
d)7AA:5Aa

could anyone pls explain me the fourth bit ...

pls do add if u have any questions related to genetics in dis blog 

all d best everyone...:)

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