1.2 Why can people look like their parents, brothers and sisters but not identical to them?
A human has 23 PAIRS OF CHROMOSOMES
Parents pass on their genes to their offspring in their sex cells.
A pair of chromosomes carries the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called ALLELES. These alleles may be the same (HOMOZYGOUS) on each pair of chromosome, or different (HETROZYGOUS) – For example to give blue eyes or brown eyes.
Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.
WHICH CHROMOSOME WE GET FROM EACH PAIR IS COMPLETELY RANDOM. THIS MEANS DIFFERENT CHILDREN IN THE SAME FAMILY WILL EACH GET A DIFFERENT COMBINATION. THIS IS WHY CHILDREN IN THE SAME FAMILY LOOK A LITTLE LIKE EACH OTHER AND A LITTLE LIKE EACH PARENT, BUT ARE NOT IDENTICAL TO THEM. THE CHILD WILL SHARE SIMILARITIES WITH ITS PARENTS DEPENDING ON WHICH CHARACTERISTICS HAVE COME FROM THE FATHER AND WHICH HAVE COME FROM THE MOTHER AND WHICH ONES ARE DOMINANT AND RECESSIVE.
An allele can be DOMINANT or RECESSIVE
- An individual with one or both DOMINANT alleles (in a pair of alleles) will show the associated DOMINANT
- An individual with one RECESSIVE allele (in a pair of alleles) will not show the associated RECESSIVE
- An individual with both RECESSIVE alleles (in a pair of alleles) will show the associated RECESSIVE
It is easiest to follow what is happening with the inheritance of gene characteristics by drawing genetic diagrams.
FAMILY TREES can be used to trace the inheritance of a characteristic and to work out who must have been carrying a faulty allele. An example of this:
When looking at the possibilities of inheriting and allele, we use a Punnett square diagram. This shows all the possible pairings of alleles from sperm and egg at fertilisation.
For example if a male with a dominant A allele and recessive a allele was to mate with the same alleles, the following diagram could be drawn:
A Punnett square diagram can also be used to represent how sex is determined. This is because one of the 23 pairs of chromosomes in a human cell is the sex chromosome. In females the sex chromosomes are the same – they are both X chromosomes. In males they are different – there is an X chromosome and a Y chromosome.
The sex of an embryo is determined by a gene on the Y chromosome called the SRY (sex-determining region Y) gene. If the gene is not present i.e. if there are two X chromosomes present, the embryo will develop into a female and ovaries will grow. If the gene is present i.e. both an X and a Y chromosome are present, then testes will begin to develop.
Six weeks after fertilisation, the undifferentiated gonads start producing a hormone called ADROGEN. Specialised receptors in the developing embryo detect the androgen. This stimulates the male reproductive organs to grow.