Meiosis and Inheritance

Meiosis and Genetic Variation

Meiosis is a type of cell division that produces four genetically distinct haploid gametes.
The process involves two rounds of division, meiosis I and meiosis II.
Prior to meiosis I, DNA in the parent cell is replicated to form chromatids.
In meiosis I, homologous chromosomes separate. Each resulting cell carries a mix of maternal and paternal genes, resulting in genetic variation.
In meiosis II, the sister chromatids separate, producing four gametes, each with a unique set of chromosomes.
Additional variation arises due to crossing over during prophase I of meiosis, where parts of non-sister chromatids exchange places.
As a result of these processes, each gamete is genetically unique, providing the basis for genetic variation within species.

Inheritance Patterns

Genes are inherited in pairs, one from each parental gamete.
A gene variant is called an allele, and individuals have two alleles (one from each parent) for each gene.
The manifestation of a gene’s effect – observable characteristics or phenotypes – depends on the two alleles possessed and how they interact.
The types of inheritance pattern include: complete dominance, incomplete dominance, codominance, and sex-linked inheritance.
Complete dominance happens when the phenotype of the heterozygote is identical to the dominant homozygote.
In incomplete dominance, a heterozygote expresses an intermediate phenotype.
In codominance, both alleles express their phenotype fully, without blending.
Sex-linked genes are located on sex chromosomes. In humans, most sex-linked genes are on the X chromosome.

Genetic Disorders

Inherited disorders are caused by faulty alleles. They may be recessive, requiring two copies of the faulty allele, or dominant, requiring only one.
Some genetic disorders include cystic fibrosis, sickle cell anaemia, and haemophilia.
Carriers are people who carry one copy of the faulty allele but do not express the associated disorder. They can, however, pass the allele on to offspring.

Punnett Squares

A Punnett Square is a diagram used to predict the outcome of a particular cross or breeding experiment.
It depicts all possible genetic combinations for the offspring, and probabilities of each.
Biologists use Punnett Squares to understand inheritance patterns, predict genetic disorders in offspring, and conduct breeding experiments.