Genes, inheritance and selection Study Guide

In biology, the terms haploid and diploid refer to the number of sets of chromosomes found within a cell's nucleus. Chromosomes are structures that contain our genetic material (DNA).

A diploid cell contains two complete sets of chromosomes, one set inherited from each parent. This means that for each type of chromosome, there are two copies: one maternal and one paternal. This '2n' state is characteristic of most somatic (body) cells in humans and many other organisms. For instance, human somatic cells have 46 chromosomes, arranged as 23 pairs; therefore, they are diploid. The presence of two sets provides genetic redundancy, meaning that if one copy of a gene has a fault, the other copy might still be functional.

Conversely, a haploid cell contains only one complete set of chromosomes. This 'n' state is typical of gametes (sex cells) such as sperm and egg cells. In humans, gametes have 23 single chromosomes, not paired. When a sperm cell (haploid) fertilises an egg cell (haploid), their nuclei fuse, combining their single sets of chromosomes to form a diploid zygote. This restoration of the diploid state is crucial for maintaining the species' characteristic chromosome number across generations. Meiosis, a specialised type of cell division, produces haploid cells from diploid cells, ensuring that the number of chromosomes is halved before sexual reproduction.

In humans, sex is determined by a pair of chromosomes called sex chromosomes. Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). During reproduction, each parent contributes one sex chromosome to their offspring. A female parent can only pass on an X chromosome to her child. A male parent, however, can pass on either an X chromosome or a Y chromosome. The combination of these chromosomes from both parents determines the sex of the offspring.

We can illustrate this using a Punnett square, which is a diagram used to predict the outcome of a genetic cross. Let's represent the female's genotype as XX and the male's genotype as XY. When drawing a genetic cross, the possible gametes (sex cells) from one parent are placed along the top, and the possible gametes from the other parent are placed along the side. Each box inside the square represents a possible genotype of the offspring.

For humans:

• Female gametes: X, X
• Male gametes: X, Y

Punnett Square: `` | X | Y | ----|-----|-----| X | XX | XY | ----|-----|-----| X | XX | XY |

From this Punnett square, we can see the possible genetic combinations for offspring:

• Two boxes show XX, which represents a female.
• Two boxes show XY, which represents a male.

This means that there is a 50% chance (or a 1 in 2 probability) for each pregnancy to result in a female (XX) and a 50% chance (or a 1 in 2 probability) for it to result in a male (XY). The male parent is responsible for determining the sex of the offspring because he can contribute either an X or a Y chromosome, whereas the female parent always contributes an X chromosome.