MENDELIAN GENETICS Biology Corsicana High School

Genetics the branch of biology that studies how traits are inherited from generation to generation

Heredity vs Environment heredity---the passing on of traits from parent to offspring environment----outside forces that act on an organism both heredity and environment affect traits

Types of Traits species traits---are the same for all members of a species individual traits---make individuals different from others in the same species

“Recent” Discoveries (within the last 100 years) Genetic information is carried on chromosomes Chromosomes are made of DNA However, 100 years before Watson & Crick, we knew the “laws” of heredity---how traits are passed on

Gregor Mendel “Father of Heredity” an Austrian monk who lived in the middle 1800s; high school teacher first person to discover the mechanism for the inheritance of traits saw regular patterns in flowers and other plants he raised

Mendel’s Experiments He chose the garden pea for his studies because: –34 different varieties –normally self-fertilizing –easily cross-fertilized by hand –could be prevented from accidental self-fertilization

Mendel’s Experiments (continued) He conducted a 7-year series of experiments on the peas His results were virtually the same for all traits he studied

Mendel’s Results Example: round x wrinkled peas –F 1 generation: all peas were round he let this generation self- fertilize –F 2 generation: 3 round : 1 wrinkled

Generations F 1 = first filial generation--- the first offspring of two “pure-breeding” plants F 2 = second filial generation- the second generation, achieved by letting the F 1 self-fertilize

Analysis of his Results Since he got virtually the same results on all traits studied, Mendel theorized that there must be laws to govern inheritance of traits He based his explanations on the laws of probability

Mendel’s Hypotheses Concept of Unit Characters--- inherited traits are controlled by factors (genes) that occur in pairs Principle of Dominance and Recessiveness---one gene in a pair may mask the other, preventing it from having effect

Mendel’s Hypotheses Law of Segregation---a pair of genes is separated in forming gametes These three laws, taken together, explain Mendel’s results

Mendel’s Hypotheses Law of Independent Assortment- --when the gene pairs (on a given pair of chromosomes) are separated, they are distributed to the gametes completely independently of the way other gene pairs (on other chromosomes) are distributed

Mendel’s Results round x wrinkled P 1 (parent) RRrr spermseggs zygote F1F1 RR rr RrRr all round

Mendel’s Results (continued) F1F1 round x round (self- fertilize) Rr R r sperms eggs RrRr zygotes 3 round : 1 wrinkled RRrRr r F2F2 r R

Genetics Vocabulary gene---a factor by which a trait is inherited allele---the alternate forms of a gene for a certain trait –example: round, wrinkled

Genetics Vocabulary (continued) dominant---a gene which will mask or cover up its allele –capital letter recessive---a gene which will be masked or covered up by its allele –lower case letter

Genetics Vocabulary (continued) homozygous---the two genes for a trait in an individual are exactly the same –example: RR, rr heterozygous---the two genes for a trait in an individual are different –example: Rr

Genetics Vocabulary (continued) genotype---the genetic makeup of an individual (the actual genes it has)---examples: RR, Rr, rr phenotype---the appearance of an organism as a result of its genotype (what it looks like)---examples: round, wrinkled, etc.

Monohybrid Cross a genetic cross dealing with one set of traits Punnett square---a grid for working genetic crosses

Example: heterozygous round x heterozygous round Rr x Rr R r r R R R R r rrr R

Genotypes 1 RR : 2 Rr : 1 rr Phenotypes 3 round : 1 wrinkled

Codominance traits in which neither allele of a pair is dominant the two genes blend together to give an intermediate trait

Incomplete Dominance One allele is not completely dominant over another In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes Example: A red flowered parent (RR) is crossed with a white flowered parent(WW) and the offspring (RW) are pink

Multiple Alleles Genes that have more than two alleles It does not mean that one individual has more than two alleles, but that more than two exist in a population Example: Coat color in rabbits and human blood type

Polygenic Traits Traits that are controlled by two or more genes “Polygenic” means having many genes Skin color is controlled by at least four different genes. That is how we have such a wide variety of colors.

Dihybrid Cross a genetic cross dealing with two traits at a time predicts what combinations of traits will be found in the offspring Mendel’s Law of Independent Assortment applies to these crosses

Key to Working Dihybrid Crosses Each gamete gets one gene from each pair