L3 Inheritance

What is Inheritance?

Inherited characteristics are because of genetic information carried by genes passed on from parents to offspring during reproduction.

Chromosomes are long pieces of coiled up DNA which contain many genes.

Sex cells ( Gametes)  carrying half the total number of chromosomes of each parent, combine during fertilisation.

In humans the sperm and egg cells carry 23 chromosomes each.

During sexual reproduction, the chromosomes combined to produce 46 chromosomes ( 23 pairs) in the embryo.

There is mixing of genes that is why the offspring looks similar but not identical to the parents.

There is more than one version of a gene.

For example, a child may inherit a tall gene from the mother and a short gene from the mother.

Genetic Mutation 

Mutation is a change in the structure of the DNA.

Mutation comes about as a result of exposure to ionisation radiation, dangerous chemicals, viruses etc.

Mutations can be in the form of genes getting deleted,  duplicated, or substituted in a DNA.

Most mutations are not harmful, but some can be which can result in cancer or genetic disorder.

A mutated cell can multiply abnormally which creates a tumour.

Some of these tumours are cancerous.

How are Characteristics Inherited?

During sexual reproduction, each parent provides half the number of chromosomes to the offspring.

Each half of the set of chromosomes contain different versions of the same gene.

For example, there are two version of genes for height which are Tall (T) and Short (t) genes.

One gene normally expresses itself  over the other whenever it is inherited.

The gene that expresses itself over the other is represented by an uppercase letter , so for the example above that is the tall gene (T)

Other gene, short gene (t), will only express itself when there are two of them.

Punnett Square

The probability of an offspring inheriting certain characteristics from its parent can be predicted using a Punnett square.

The Punnett square shows the probability of offspring of a tall father who is a carrier of short genes and a mother who is short having a child is tall or short.

To complete the Punnet square you match a box to the other in order.

The probability that the child will be tall is 50% and the probability that the child will be short is 50%.

Sex Determination

The sex determining chromosomes in the human sperm is  XY  and that in egg cell is XX.

The probability of having a male or female is ½ or 50%.

Task 1

Complete the Punnett square to show the probability of producing smooth peas when two smooth pea plants which carries a wrinkled gene ( Ss)  are crossed.

1. Complete the Punnett square
2. What is the probability of producing wrinkled peas?
3. What is the probability of producing smooth peas?
4. If 2000 peas were produced, how many were wrinkled and how many were smooth?

Answers 

1. 

2. 25%

3. 75%

4.  500 ( 25%  x  2000) wrinkled peas and  ( 75% x 2000) 1500 smooth peas