Determining of the first generation genotype and phenotype (I-st Mendel’s Law)

4. In Holstein cattle the spotting of the coat is due to a recessive gene while a solid–coloured coat is dominant. What types of offspring might be produced by a cross between two spotted animals? Show how you reach your conclusion. The gene P is responsible for coat pattern.

Let P be the dominant allele for a solid coloured coat.

Let p be the recessive allele for a spotted coat.

Parents: pp x pp (no P present)

Punnett table

sperm sperm

	p	p
p	pp	pp
p	pp	pp

egg

egg

All of the offspring of two spotted animals are also spotted.

5. In cats the gene for short hair is dominant over the gene for long hair (angora). A short–haired tom is mated with an Angora female. She bears eight kittens, six short–haired and two with long hair. How do these numbers compare with the expected ratio? If you mated these same cats four more times and obtained a total of forty offspring, would you expect the results to be a closed approximation of the expected ratios? Explain.

Let gene H control the length of the cat’s hair

Let H be the dominant allele for short hair.

Let h be the recessive allele for long hair.

Short–haired tom: H_ (we don’t know the second allele for sure)

Angora female: hh

Two of their offspring are long–haired with a genotype of hh. Therefore, each parent contributed an h allele and therefore the tom cat father has the genotype Hh.

Punnett table

eggs (all the same)

	h	h
H	Hh	Hh
h	hh	hh

sperm

sperm

Expected ratio is 1 Hh : 1 hh

Actual ratio is 3 Hh : 1 hh

Over a long range of matings, the actual ratio should approach the expected ratio.

6. In summer squash, white coloured fruit is dominant over yellow. If you place pollen from a yellow–fruited plant on the pistil of a hybrid white–fruited ( heterozygous ) plant, what type of seeds would you expect from the seed which come from this cross? Let the gene C control fruit colour in summer squash.

Let C be the dominant allele for a white–coloured fruit.

Let c be the recessive allele for a yellow–coloured fruit.

Punnett table

eggs eggs

	C	c
c	Cc	cc

Pollen

Cc : hybrid white–fruited plant

Cc : yellow–fruited plant

You can expect hybrid white–fruited plants and yellow–fruited plants in a 1 : 1 ratio

7. In Drosophila, vestigial wings and ebony colour are due to two separate recessive genes. The dominant alleles are normal (long) wings and normal (gray) body colour. What type of offspring would you expect from a cross between a homozygous vestigial ebony female and a normal double homozygous (long–winged, gray–bodied) male? If the F₁ are allowed to breed among themselves what types of offspring would you expect in the F₂? Show complete genotype and phenotype of both generations.

This is a dihybrid cross.

Gene L controls wing length and gene G controls body colour.

Let L be the dominant allele for normal long wings.

Let l be the recessive allele for vestigial wings.

Let G be the dominant allele for normal gray body colour.

Let g be the recessive allele for ebony body colour.

Homozygous vestigial ebony female : llgg

Normal (long–winged, gray–bodied) male : LLGG

Parents : llgg x LLGG

Punnett table

eggs

	lg
LG	LlGg

sperms

Offspring from this mating are long–winged, gray–bodied Drosophila.

We raise an F₂ by mating the F₁ among themselves: LlGg x LlGg