genetic disorders

Albinism is absence of pigmentation melanin in human skin, animals or plants. This pigmentation is responsible for dark colour of the skin. As a result the person has white hair, pink eyes and light skin. In plants they are characterized by lack of chlorophyll.

Examples

What will be the result of a normal man who married an albino woman?

Solution: Let gene for normal be A and albino be a

Phenotype – all are normal (heterozygous)

What would be the result of a cross between heterozygous parents?

Genotype: AA, Aa and aa

Phenotype: normal, carrier and albino

What would be the result of a cross between a heterozygous parent with an albino parent?

Solution:

Gene: Aa heterozygous parent

aa albino parent

Genotypes: Aa and aa

Phenotypes: half normal/carriers and albinos

What would be the result of a cross between a heterozygous parent and a homozygous normal parent?

Solution: Heterozygous Aa, homozygous AA

Genotypes: AA, Aa

Phenotypes: all are normal (normal, carriers)

Achondroplasia is a disorder that is characterized by a shortened body, legs and hands. It is controlled by a dominant gene. Individuals with this disorder are homozygous dominant or heterozygous. Homozygous recessive are perfectly normal.

Examples

1. What would be the result of a normal man who married an achondroplasia woman?

Solution:

Genes for normal man: aa

Genes for achondroplasia woman: AA

Phenotypes: all are achondroplasia

Genotypes: Aa

2. What would be the result of a cross between an achondroplasia woman who is homozygous and an achondroplasia man who is heterozygous?

Solution:

Genotypes: AA, Aa

Phenotypes: all achondroplasia

3. What would be the result of a cross between heterozygous parents?

Solution:

Phenotypes: 3 Achondroplasia, 1 normal

Genotypes: AA, Aa, aa

Phenotypic ratio: 3:1

Genotypic ratio: 1:2:1

Haemophilia is a hereditary trait characterized by delayed blood clotting. The result is prolonged bleeding; even small injuries can lead to death. The haemophilic girl rarely lives beyond puberty because of excessive menstrual bleeding. It causes high mortality rate.

It is controlled by a recessive gene. Heterozygous are normal/carriers but homozygous individuals are haemophilic.

Examples

If a normal man married a haemophilic woman, the offspring would be:

Solution:

Let genotype for the man $X^H Y$ and woman $X^h X^h$

1. A haemophilic man will be $X^h Y$
2. Haemophilic female will be $X^h X^h$
3. H – not suffering from haemophilia while h – haemophilic

Colour blindness is a hereditary trait characterized by inability to detect certain colours of the spectrum. The common colour blindness is inability to distinguish between red and green.

A recessive gene controls it. Homozygous individuals are colour blind while heterozygous are normal or carriers.

e.g. If a colour blind man marries a normal woman, the offspring will be as follows.

Let B = normal

b = colour blind

This is a genetic disorder, which makes the red blood cell acquire sickle shape under certain conditions. It may occur when certain diseases attack the person, e.g. malaria. Also when oxygen tension in the atmosphere is very low. The sickled cells ability to carry oxygen is reduced.

A recessive gene controls it. Homozygous individuals are sickled while heterozygous individuals are normal/carriers.

NOTE:

HbA – perfect normal

HbS – sickle cell trait

If a carrier man marries a carrier woman the offspring will be – sickle cell anaemia

Tongue rolling is a simple genetic trait often used as an example in teaching inheritance patterns. It refers to the ability to roll the tongue into a tube-like shape, a trait thought to be controlled by a single gene with two alleles: one for the ability to roll the tongue (dominant) and one for the inability to roll the tongue (recessive).

Example

Let's consider a cross between two individuals:

One parent is heterozygous (Tt) for tongue rolling (can roll their tongue).

The other parent is homozygous recessive (tt) for tongue rolling (cannot roll their tongue).

Cross: Tt × tt

The possible gametes from each parent:

The Tt parent can contribute either T or t.

The tt parent can only contribute t.

Punnett Square for the Cross

	t	t
T	Tt	Tt
t	tt	tt

Resulting Offspring Genotypes

Tt (heterozygous) — 50% chance

tt (homozygous recessive) — 50% chance

Phenotypes of the Offspring

Tt (heterozygous) — These offspring will be able to roll their tongue because the T allele is dominant.

tt (homozygous recessive) — These offspring will not be able to roll their tongue because they have two copies of the recessive allele.