Introduction to Genetics Chapter 11

What is inheritance? Genetics – the scientific study of heredity. Gregor Mendel – an Austrian monk who observed patterns of inheritance in pea plants. Why peas? Breeding could be easily controlled (pollen and eggs), they grew fast, and had variable genetic traits (characteristics).

Peas had varying traits: Round, wrinkled, green, yellow, tall, short, axial flowers, terminal flowers, etc…

Mendel’s observations of peas: Some plants were true-breeding – they always produced the same offspring. Always tall, green… Pea plants could be selectively bred by brushing on or inhibiting pollen. Certain traits would “disappear” for a generation, then return again. Repeating patterns emerged.

Mendel’s observations P generation – parent generation. F1 generation – first filial, offspring of P generation. Hybrid – offspring produced by parents of different traits.

Patterns emerging… When Mendel bred the purple and the white, all the F1 generation were purple. The white disappeared. White flower trait is recessive. All offspring were purple. Purple flower trait was dominant. Different versions of a gene are called alleles.

Would the recessive trait return? Mendel bred the F1 generation to produce an F2 generation: flower color, pod color, shape…The recessive returned. What ratio was observed?

Mendel’s conclusions: Each trait has two alleles (versions). Plant height: Tall (T) or Short (t).. These alleles segregate, or separate when gametes (egg and sperm) are formed. TT makes T gametes, Tt makes T gametes and t gametes.. The allele from a sperm and the allele from the egg is the organism’s genotype. TT, Tt, or tt.. What the organism looks like is its phenotype. Tall plant, short plant..

Genotype, phenotype, homozygous (same) and heterozygous (different) traits

Probability and Punnett Squares: When gametes are formed, there is a 50/50 chance that it will get one version of a trait. Like a coin flip! The different possible offspring, and ratios of genotypes produced in a cross can be shown using a Punnett square. Did the 3:1 ration for Mendel’s F1 cross make sense?

A Monohybrid (one trait) cross:

Dihybrid (two traits) cross, Trihybrid… In reality, all traits are passed on to the gametes. Most traits go into the gametes independent of each other. Example: A homozygous tall, yellow plant (TTYY) produces only the following gamete: TY. Another example: a heterozygous tall, yellow plant TtYy can produce the following gametes: TY, Ty, tY, ty.

Try a Dihybrid cross:

Other modes of inheritance…. Incomplete dominance – the dominant gene does not completely show The heterozygous is in-between Japanese four-o-clocks, snapdragons

Other modes… Codominance – both alleles show up in the phenotype. Examples: coat color in cows, speckled hens, sickle-cell trait, blood type… Multiple alleles – more than two alleles for a trait.

Meiosis – Gamete production! In order to make an individual with the full component of chromosomes (diploid), gametes must be created (haploid). Gametes are sex cells with half the amount of chromosomes. We have 23 homologous (same) pairs of chromosomes, total = 46. Gametes (egg or sperm) have 23.

Meiosis… Meiosis consists of 2 divisions of a diploid or 2N cell, to create 4 haploid cells (1N each). Before the first division, the chromosomes replicate to form the tetrad (X X figure). The homologous pairs and their copies go through crossover. Crossover produces new combinations of alleles, It makes you unique!

Crossover… Crossover happens in prophase I, and sections of one homologue are traded with sections of another. Alleles are shuffled from one to another. This is a random event, with an infinite number of combinations!

Crossover… If genes are close together on an arm of a chromosome, they are more likely to transfer together. Because of this, they are said to exhibit linkage. This helps scientists map the genes on a chromosome!

Gene maps

Meiosis continued… Meiosis I – chromosomes duplicate, crossover occurs, IPMAT, results in 2 diploid cells. Meiosis II – PMAT, results in haploid cells. Note that these 4 gametes are each genetically unique! Non-disjunction - if the chromosomes fail to separate properly, disorders occur.

Meiosis