Genotype and phenotype

The genotype of an organism is identical to the:

• phenotypic expression of the gene concerned.

• combination of alleles at the gene concerned.

• karyotype

• genome

The gene for seed colour in the common edible pea (Pisum sativum) is autosomal. It has two alleles: G and g.

What is the genotype of a pea plant that is heterozygous for seed colour?

• Gg

• X^GX^g

• GG or gg

• X^GY or X^gY

For the two sexes of a mammal, all possible genotypes for a gene with two alleles (A and a) can appropriately be identified as

• homozygous AA or aa in males for an autosomal gene.

• AA, Aa and aa in males for an X-linked trait.

• homozygous X^aX^a or X^AX^A in females for an X-linked trait.

• homozygous AA or aa or heterozygous Aa in females for an autosomal gene.

In humans, the TYR gene produces the enzyme, tyrosinase which controls pigment production. People who lack tyrosinase cannot produce pigment. They have albinism and have no pigmentation in their hair, skin or eyes.

There are two alleles for the TYR gene: A and a. Heterozygotes for TYR can produce tyrosinase and have normally pigmented skin, hair and eyes.

From this information it can be concluded that:

• albinism is dominant over normal pigmentation.

• albinism is recessive to normal pigmentation.

• production of tyrosinase is needed to show albinism.

• the genotype of an albino is AA.

In rabbits, the B gene locus has two alleles: B and b.

B produces a black coat and b produces a chocolate (dark-brown) coat. A rabbit with the genotype Bb has a black coat.

Which statement is correct regarding the coat colour of rabbits?

• b is the recessive phenotype.

• A heterozygote rabbit has the genotype BB.

• A rabbit with the genotype bb has a chocolate coat.

• Each somatic cell of a rabbit has either a B or a b allele for coat colour.

In humans, the CBD gene controls colour vision. The gene locus is on the X chromosome.

There are two alleles, X^B and X^b.

Individual 1 is a male with normal colour vision.

Individual 2 is a male with defective colour vision.

From this information it can be stated that:

• normal colour vision is recessive to defective colour vision.

• there are three possible genotypes in males with respect to the CBD gene.

• individuals 1 and 2 are both homozygous at the CBD locus.

• the phenotype of a heterozygous female will confirm the dominant phenotype.

Petal colour in a species of flowering plant is a monogenic trait.

It is observed that, in this species, only two petal colours occur, namely red and white.

It is reasonable to conclude that:

• petal phenotype is determined by two genes, red and white.

• petal phenotype is determined by different alleles of one gene.

• petal phenotype is under environmental control.

• red is dominant to white

A gardener obtained six identical pots and filled them with soil of different PH. In each pot he planted a stem-cutting, taken from the same parent hydrangea plant.

Several months later, flowers appeared on all of the cuttings. The flowers in each pot had a different colour as shown in the table.

• acidic soils promote pink colour in hydrangea flowers.

• the plant in each pot had a different genotype.

• the difference in flower colour must be a result of genes switching ‘on’ or ‘off’.

• if a hydrangea cutting from the same plant is grown in soil of pH 5.2 it would produce bluish-purple flowers.