Sex determination in humans
Sexual reproduction
Sexual reproduction is the formation of a new organism by combining the genetic material of two organisms.
During this process the The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei. of the male and female sex cells, called Sex cell (sperm in males and ova/eggs in females)., fuse to create a A fertilised egg cell.. This process is known as The joining of a male and female gamete..
Each gamete usually contains half the number of The structure made of DNA that codes for all the characteristics of an organism. and is described as A sex cell (gamete) that contains one set of chromosomes..
When the male and female gametes combine, they usually restore the full complement of chromosomes - or A cell that contains two sets of chromosomes. number - to produce a human embryo.
The gametes in animals are sperm and eggs.
The gametes in flowering plants are pollen and eggs.
The offspring produced in sexual reproduction are To do with the genes that an organism contains. different to each other and the parents. This process results in Difference between individuals, distance from the norm. as it involves the mixing of genetic information.
The advantages of sexual reproduction are:
- it produces variation in the offspring
- the A type of organism that is the basic unit of classification. Individuals of different species are not able to interbreed successfully. can adapt to new environments due to variation, which gives them a survival advantage
- a disease is less likely to affect all the individuals in a population
- humans can speed up natural selection through An artificial process in which organisms with desired characteristics are chosen as parents for the next generation., which can increase food production
Sexual reproduction - conception
Sex determination in humans
Human body cells usually have 23 pairs of chromosomes in the nucleus.
22 pairs control most of the characteristics. Pair 23 are the sex chromosomes. They carry The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. that determine an embryo's sex - whether offspring are biologically male or female:
- typically males have two different sex chromosomes, XY
- typically females have two X chromosomes, XX
In humans, some genetic conditions occur when there is a variation in the number of chromosomes. For example, people with Down’s Syndrome have 47 chromosomes in their body cells rather than 46.
Chromosomes from biological males
These diagrams are known as human karyotypes, and show all the chromosomes, cut out from a A photograph taken of a microscopical image. and aligned in pairs.
The blue box shows the two sex chromosomes - these are different sizes, and are an X (larger chromosome) and a Y (smaller one).
Chromosomes from biological females
The red box shows the two sex chromosomes - these are the same size, both two X, larger chromosomes.
Genetic diagram
A genetic diagram, like a Punnett square, shows how biological sex is inherited.
The two typical combinations are:
- an X chromosome from the mother and an X chromosome from the father - producing a girl (XX)
- an X chromosome from the mother and a Y chromosome from the father - producing a boy (XY)
The ratio of female to male offspring is 1:1, ie half of the offspring will be girls and half will be boys ie on average. This can also be converted into a probability of 50% (XX) and 50% (XY).
