Pearson Edexcel GCSE (9-1) Biology revision notes
Genetics Revision Notes
Review revision notes for Genetics in Edexcel Biology.
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Genetics
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Inherited Characteristics and Allelic Variation Revision Notes
What are Alleles?
Alleles are different versions of the same gene. For example, a gene for eye colour might have an allele for blue eyes and an allele for brown eyes. Individuals inherit two alleles for most genes, one from each biological parent.
Sources of Allelic Variation
The primary sources of variation in inherited characteristics are the different combinations of alleles passed down from parents during sexual reproduction. This random assortment of alleles during gamete formation and their subsequent fusion in fertilisation ensures offspring are genetically distinct from their siblings and parents. Mutations can also create new alleles, further contributing to variation.
Homozygous vs. Heterozygous
An individual is homozygous for a gene if they inherit two identical alleles (e.g., two alleles for brown eyes). They are heterozygous if they inherit two different alleles (e.g., one for brown eyes and one for blue eyes). The interaction between these alleles (dominance, recessiveness) determines the expressed trait.
Meiotic Cell Division and Gamete Formation Revision Notes
Purpose of Meiotic Division
Meiosis is a specific type of cell division necessary for sexual reproduction. Its primary role is to produce gametes (sex cells like sperm and eggs) that have half the number of chromosomes of the parent cell, ensuring that when two gametes fuse during fertilisation, the resulting zygote has the correct diploid chromosome number.
Outcomes of Meiosis
Meiotic division results in four daughter cells from one parent cell. Each daughter cell is haploid, meaning it contains only one set of chromosomes. These daughter cells are also genetically distinct from each other and the original parent cell, which generates genetic variation within a species.
Haploid Gametes
The four cells produced by meiosis are called gametes. They are haploid, meaning they contain half the normal number of chromosomes found in somatic (body) cells. This reduction is essential to maintain the correct chromosome count across generations after fertilisation.
ABO Blood Group Inheritance Revision Notes
Multiple Alleles in ABO Blood Groups
Blood group inheritance involves multiple alleles, meaning there are more than two possible alleles for a single gene in the population. Specifically, there are three alleles responsible for the ABO blood types: Iᔧ (produces A antigen), Iᔥ (produces B antigen), and i (produces no antigen).
Codominance and Dominance in ABO Blood Groups
The relationship between these alleles demonstrates both codominance and simple dominance. Alleles Iᔧ and Iᔥ are codominant, meaning that if an individual inherits both, both antigens (A and B) are expressed, resulting in blood type AB. Both Iᔧ and Iᔥ are dominant over the i allele, meaning that if i is paired with either Iᔧ or Iᔥ, only the A or B antigen will be expressed, respectively. The i allele is recessive, so individuals with genotype ii have blood type O.
Genotypes and Phenotypes of ABO Blood Groups
The possible genotypes and their corresponding phenotypes are: IᔧIᔧ or Iᔧi result in blood type A. IᔥIᔥ or Iᔥi result in blood type B. IᔧIᔥ results in blood type AB. ii results in blood type O.
Sex Determination at Fertilisation Revision Notes
Chromosomes and Sex
Human body cells contain 23 pairs of chromosomes, totalling 46. One pair consists of the sex chromosomes, which determine an individual's biological sex. Females have two X chromosomes (XX genotype), while males have one X and one Y chromosome (XY genotype). The other 22 pairs are called autosomes and carry genes for other characteristics.
Gamete Formation and Fertilisation
During gamete formation (meiosis), the pairs of chromosomes separate. A female produces egg cells, and each egg contains a single X chromosome. A male produces sperm cells; approximately half of these sperm will carry an X chromosome, and the other half will carry a Y chromosome. Sex is determined at fertilisation. If an X sperm fertilises the egg (X), the zygote will be XX (female). If a Y sperm fertilises the egg (X), the zygote will be XY (male). This gives a 50% probability for either sex.
Genetic Diagrams for Sex Determination
Genetic diagrams, such as Punnett squares, can illustrate sex determination. Parental genotypes are XX (female) and XY (male). Gametes for female are X and X. Gametes for male are X and Y. The potential offspring genotypes are XX and XY, each with a 50% probability, demonstrating equal chances for male or female offspring.
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