- Define inheritance as the transmission of genetic information from generation to generation
Inheritance is the transmission of genetic information from generation to generation.
- Define chromosome as a thread-like structure of DNA, carrying genetic information in the form of genes
A chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes.
- Define gene as a length of DNA that codes for a protein
A gene is a length of DNA that codes for a protein.
- Define allele as a version of a gene
An allele is a version of a gene.
- Describe the inheritance of sex in humans with reference to XX and XY chromosomes
Sex gametes in humans (i.e. the sperm and egg) are formed through meiosis – a type of cell division in which a diploid parent cell divides into haploid daughter cells. (Cell division is covered in the next section).
This means each sex gamete has only one set of chromosomes.
There are two types of sex chromosomes in humans – the X chromosome and the Y chromosome.
Also, females have two X chromosomes in each diploid cell, so they are XX and males have one X and one Y chromosome, so they are XY.
This means that when eggs (the female gamete) are formed, the only sex chromosome they can have is X, so all eggs have the X chromosome.
When sperm are formed, however, they have a 50/50 chance of getting either the X chromosome or the Y chromosome.
In other words, half of the produced sperm will have an X chromosome and the other half will have a Y chromosome.
This means that it is equally likely that a sperm with an X chromosome and a sperm with a Y chromosome will fertilise the egg, so the chances of the zygote being male or female are equal.
- Define a haploid nucleus as a nucleus containing a single set of unpaired chromosomes e.g. in gametes
This is a pretty self-explanatory point! Simply learn the definition 😊
- Define a diploid nucleus as a nucleus containing two sets of chromosomes, e.g. in body cells
Once again, all you have to do is learn the definition!
- State that in a diploid cell, chromosomes are arranged in pairs and in a human diploid cell there are 23 pairs
Another self-explanatory point – in a diploid cell, chromosomes are arranged in pairs and in a human diploid cell there are 23 pairs.
Notes submitted by Sarah
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