Genetics

Review: Chromosome: made up of tightly coiled DNA

Humans have 23 pairs of chromosomes in each of their somatic cells (also known as body cells). Of each pair, you get one from your mom and one from your dad. These sets of chromosomes are homologous. Homologous chromosomes: have the same structural features and pattern of genes, but may have different alleles.

The Discovery of Genetics Genetics is the study of heredity, or how traits are passed down from one generation to the next. A trait is a specific characteristic that is expressed by genes. The “father” of genetics is Gregor Mendel. He was an Austrian monk who studied how traits are inherited using garden peas.

During sexual reproduction, a sperm cell and an egg cell join in a process called fertilization. In pea plant flowers, the male part of each flower produces pollen (containing sperm), and the female part of each flower produces egg cells. Mendel had true-breeding pea plants that, if allowed to self- pollinate, would produce offspring identical to themselves. Circle the sperm producing part of the flower and the egg producing part of the flower

Using cross- pollination, Mendel was able to produce seeds that had two different parents with two different observable traits. The offspring of crosses between parents with different traits are called hybrids.

Mendel’s Results P stands for the parental generation, and F1 stands for “first filial” generation.

Mendel’s Conclusions Mendel concluded that biological inheritance is determined by factors that are passed from one generation to the next. We call these genes: a sequence of DNA that codes for a protein and thus determines a trait Each of the traits Mendel studied was controlled by one gene that occurred in two different forms that produced different characteristics for each trait. We call these alleles: the different forms of a gene

The Principle of Dominance The principle of dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait. Dominant alleles are represented by a capital letter (T). An organism with a recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present. Recessive alleles are represented by a lowercase letter (t). TT Tt tt

TT Tt tt Genotype: the genetic makeup for a trait Example: TT, Tt, tt Phenotype: the physical feature resulting from a genotype Example: tall, short Homozygous genotype: gene combination involving two of the same alleles; either both dominant or both recessive Also called pure or pure-breed Example: TT, tt Heterozygous genotype: gene combination involving two different alleles; one dominant and one recessive Also called hybrid Example: Tt TT Tt tt

The Law of Segregation During gamete (sperm or egg cell) formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. In Mendel’s experiments, each F1 plant produced two types of gametes- those with the allele for tallness and those with the allele for shortness.

Law of Independent Assortment Alleles for different traits are distributed to gametes (sex cells) independently of one another. Genes that segregate independently do not influence each other’s inheritance. Mendel discovered this by doing a two factor cross between a plant that produced round (RR) yellow (YY) peas with a plant that produced wrinkled (rr) green (yy) peas. RRYY x rryy