Biology 120 Notes (Part 18) Continuing Genetics

What Traits Mendel Studied and His Genetic Terms

• Mendel worked with pea varieties that differed in seven easily recognizable traits, this can also be referred to as an individual’s ==phenotype== (a trait that can be observed.)
• ==Genotype== refers to the two alleles an individual has for a specific trait. The two categories of genotypes are homozygous and heterozygous.
• Mendel’s pea population had two distinct phenotypes for each of the seven traits.
• A ==trait== is a variant for a character, for example: brown.
• ==True breeding== is a term that describes how all of the offspring are of the same variety.
• A ==monohybrid cross== describes the mating between two parents that differ in only one trait.
• We label the different generations of a cross as: ==P generation== (parents), ==F1 generation== (first filial generation), and ==F2 generation== (second filial generation).
• ==Testcross== is a process that is used to assess whether an individual displaying a dominant trait is heterozygous or homozygous

The Monohybrid Cross

• Mendel crossed plants with round seeds and plants with wrinkled seeds.
• He observed that all of the F1 offspring had round seeds, and this contradicted the hypothesis of blending inheritance.
• The genetic determinant for wrinkled seeds seemed to have disappeared.
• Mendel allowed the F1 progeny to self-pollinate and the wrinkled seed trait reappeared in the F2 generation.

Mendel’s Law of Segregation

• Mendel’s ==Law of Segregation== how each individual has a pair of factors for each trait called alleles. The alleles segregate during gamete formation.
• each gamete contains only one allele from each pair of factors.
• Fertilization gives the offspring two factors for each trait.

Mendelian Genetics

• Each trait in a pea plant is controlled by two alleles (alternate forms of a gene).
• Alleles can be dominant or recessive.
• A dihybrid cross was performed involving pea plants that differed in two traits.
• ==The two possible results were:== the F2 generation with two phenotypes (identical to the parents) or the F2 generation with four phenotypes.

Mendel’s Law of Assortment

• Each pair of alleles segregates independently of the other pairs.
• All possible combinations of alleles can occur in the gametes.
• The dihybrid cross always yields a phenotypic ratio of 9:3:3:1 in the F2 generation.

Human Genetic Disorders

• Genetic disorders can be either autosomal or sex-linked.
• ==Autosomal disorders== affect autosomes and ==sex-linked disorders== affect the X and Y chromosomes.
• To have an ==autosomal dominant disorder==, the individual who has it would either have to have AA or Aa.
• Examples of autosomal dominant disorders would be Osteogenesis Imperfecta (“brittle bone disease”) and Hereditary Spherocytosis (causes abnormal red blood cells that hurt easily).
• To have an ==autosomal recessive disorder==, the individual who has it would have homozygous recessive alleles (aa).
• Examples of autosomal recessive disorders would be Methemoglobinemia and Cystic Fibrosis.

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