6.6 Meiosis and Genetic Variation

Let’s Visit the Nursery!• What did we learn from our little Potato Lab darlings?• What were some examples of genetic variations that

were part of your Baby lab?• What traits were displayed?

6.6 Meiosis and Genetic Variation

KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.

6.6 Meiosis and Genetic Variation

Let’s Talk Terms!

• Homozygous- two of the same alleles (versions of a gene) at a specific locus on a chromosome

• Heterozygous- two different alleles at a specific locus• Complete dominance- a kind of dominance when the

dominant allele completely masks the effect of the recessive allele in a heterozygous condition.

• Incomplete dominance- a kind of dominance occurring in heterozygotes when the dominant allele is only partially expressed, and usually resulting in an offspring with an intermediate phenotype. Example: Red flower = dominant but partial crossed with a White flower = recessive. Yields a Pink flower.

6.6 Meiosis and Genetic Variation

What? More Terms?• Codominance- a condition where the alleles of a gene

pair in a heterozygote are fully expressed resulting in offspring with a phenotype that is neither dominant nor recessive. A typical example showing codominance is the ABO blood group system. For instance, a person having A allele and B allele will have a blood type AB because both the A and B alleles are codominant with each other.

• Polygenic trait-Polygenic traits are controlled by two or more than two genes (usually by many different genes) at different loci on different chromosomes. These genes are described as polygenes. Examples of human polygenic inheritance are height, skin colour and weight. Polygenes allow a wide range of physical traits. For instance, height is regulated by several genes so that there will be a wide range of heights in a population

6.6 Meiosis and Genetic Variation

Sexual reproduction creates unique combinations of genes.

• Sexual reproduction creates unique combination of genes.– independent assortment of chromosomes in meiosis– random fertilization of gametes

• Unique phenotypes may give a reproductive advantage to some organisms.– More skillful

hunters– Mothers who

produce more

milk

6.6 Meiosis and Genetic Variation

Fertilization (PN)

Fertilization is• Random• Increases unique combinations of genes• In humans, the chance of getting any one combination of

chromosomes from any one set of parents is one out of • 223 x 223, or one out of over 64 trillion combinations

6.6 Meiosis and Genetic Variation

Meiosis (PN)

• Independent assortment of chromosomes: – Increases unique combinations of genes– Homologous chromosomes pair randomly along the

cell equator– In human cells, about 223, or 8 million different

combinations of chromosomes can result

6.6 Meiosis and Genetic Variation

Crossing over (recombination) during meiosis further increases genetic diversity. (PN) • Crossing over is the exchange of chromosome

segments between homologous chromosomes.– occurs during prophase I of meiosis I– results in new combinations of genes

6.6 Meiosis and Genetic Variation

Genetic Linkage (PN)• Chromosomes contain many genes.

– The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.

– Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.

• Genetic linkage allows the distance between two genes to be calculated.