I/GCSE Biology - Genetic Diagrams

In I/GCSE Biology, genetic diagram is really important and it's commonly asked in the exam!

• 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.

Do you remember the sex determination in I/GCSE Biology?

• The sex of an embryo is determined by a gene on the Y chromosome called the SRY (sex-determining region Y) gene.

1. 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.
2. 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.

And we're done with this topic! Well Done!

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Drafted by Alyssa (Biology)

 References:

1. "Genetic diagram". Photo from the internet.
2. "Punnet suqare digram 1". Photo from the internet.
3. "Punnet suqare digram 2". Photo from the internet.
4. "Fertilisation". Photo from the internet.
5. "Genetic Mechanisms of Sex Determination", https://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/117894590/7_2_1.jpg
6. "Sex determination and sexual differentiation of the brain", https://www.researchgate.net/profile/Alexandra-Turano/publication/326139897/figure/fig1/AS:662510894055424@1534966314579/Sex-determination-and-sexual-differentiation-of-the-brain-Sex-determination-occurs-at.png