Chapter 11 Introduction to Genetics Gregor Mendel

A brief history Gregor Mendel Breeding experiments using garden peas (1850s) Established basic principles of heredity Father of genetics

Basic vocabulary 1) Dominant 2) Recessive 3) Alleles Trait that is expressed (by covering another) 2) Recessive Trait that is hidden (may be covered by another) 3) Alleles All genes for a specific trait

Dominant and recessive traits The traits that seem to mask other traits when present are called dominant traits. The traits that seem to be hidden in the presence of dominant traits are called recessive traits.

5) Heterozygous (hybrid) 4) Homozygous (pure) Both genes for a trait are the same 5) Heterozygous (hybrid) Both genes for a trait are different

Homozygous vs. Heterozygous Homozygous: inheriting two of the same alleles for a trait (AA tall/tall or aa short/short) Allele: a variety of a trait Heterozygous: inheriting different allele for a trait (Aa, tall/short) Mendel concluded that each trait has two alleles that determines its appearance.

Phenotype? Genotype? 6) Phenotype Actual expression of a gene (words describing) 7) Genotype Actual genes present (symbols used) Phenotype? Red or White Genotype? RR or Rr or rr

8) Mendel’s crosses: P = original parents F1 = first filial (offspring) of parents F2 = second filial (offspring of F1)

What did Mendel observe? When a true-breeding (homozygous) tall plant is crossed with a true-breeding short plant in the P generation, the F1 height trait is always predictable. 100% are tall plants. P generation F1 F2

Law of Segregation Mendel concluded only one allele is passed from parent to offspring for each trait. F1 plants must be heterozygous because the P generation only passed on one tall allele and one short allele. The F1 plant will then pass on to its offspring either a tall or a short allele, never both.

What happens when the F1 tall plants are crossed together? Mendel observed that the F2 generation, the offspring of F1 plants, are always in a fixed ratio of 3:1 tall:short. Why? P generation F1 F2

9) Punnett Squares Mathematical grid showing probable or possible offspring from a cross (phenotype & genotype ratios) Monohybrid cross = working with only one trait

Punnett square Tt x Tt Punnett created a table to show the average inheritance pattern of one offspring.

Pea traits that Mendel identified Through multiple crosses, Mendel determined that all these traits displayed a mathematical predictability for inheritance. Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall

Law of independent assortment Because organisms are made up of more than one trait, Mendel concluded that the inheritance of one trait does not influence the inheritance of a second trait. Example: Height of the pea plant does not influence the color of the peas Height is independently assorted from color.

10) Dihybrid cross working with two traits (gives twice as many gametes possibilities, so 4 times as many offspring) classical ratio of 9:3:3:1

Using dihybrid crosses to show independent assortment A smooth, yellow pea (RrYy) can pass on these combinations of genes to its offspring: RY, Ry, rY, or ry.

11) incomplete dominance Both alleles for a trait blend together creating a new expression in the heterozygous condition examples: snapdragons

Variations on Mendel Incomplete dominance: the heterozygous genotype shows a blend of the two parents and not the dominant allele

12) co-dominance Both alleles for a trait show up equally Examples: roans, “checkered” chickens

Variations on Mendel Codominance: the heterozygous genotype shows both inherited alleles Example of roan horse coat: AA (dark red) x aa (white)  Aa (dark red and white)

13) multiple alleles More than two alleles for a trait Examples: coat color of rabbits Key C = full color Cch = chinchilla Ch = Himalayan C = albino CC, Ccch, Cch, or Cc cchch, cchcch, or cchc chc or chch cc

Variations on Mendel Multiple alleles: when there are more than two alleles that code for a trait Example: ABO blood type A type = AA or Ao B type = BB or Bo O type = oo AB type = AB

Blood typing

14) polygenic inheritance Many genes affect the expression of the trait Examples: skin, eye, & hair colors

Variations on Mendel Polygenic trait: when more than one gene codes for a particular trait Example: fur color, human height, human skin color, eye color

Modernizing Mendelian genetics DNA is the basis for inheritance. DNA are coiled into chromosomes. Parts of the DNA that code for a trait are called genes. Some genes have only two alleles and other have more. Gene for hairline Allele: A Genotype: Aa Gene for hairline Allele: a

Homologous Chromosomes. (homologues) Chromosome pairs or mates (similar in size, shape, info) 0ne from each parent Humans have 23 pair (total = 46)

Meiosis A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By the end of meiosis II, the diploid (2n) cell that entered meiosis has become four haploid (n) cells.

Meiosis Production of haploid gametes Similarities to Mitosis: Same basic stages, except occur twice

Gametes Haploid (n or 1n) Reproductive cells (eggs & sperm) A cell with one of each type of chromosome or half the total number

Result is “reduction division” into 4 haploid cells instead of 2 diploids In males = all 4 become sperm In females = only 1 egg; other “polar bodies” disintegrate

Meiosis 1

Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells.

Zygote (fertilized egg) first cell of an organism with a complete set of chromosomes Diploid (2n) Cell with a full set (two of each type) of chromosomes

Oocyte or Spermatocyte (egg cell) (sperm cell) This cell that can undergo meiosis originally has 6 chromosomes and has replicated to 12 chromosomes in preparation for meiosis.

Differences: Homologues remain together during prophase I Crossing over (when chromatids exchange pieces) occurs during metaphase I

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Prophase I: homologous chromosomes pairing into tetrads

Metaphase I: tetrads align, along the metaphase plate

Anaphase I: homologous chromosomes separate from the metaphase plate If chromosomes do not properly separate, this is called nondisjunction. Nondisjunction leads to trisomy and monosomy disorders.

Telophase I: membranes form around the separated homologues

Prophase II: spindle fibers bind to the sister chromatids of each chromosome

Metaphase II: chromosomes align along the metaphase plate

Anaphase II: sister chromatids separate to opposite poles

Telophase II: nuclear membrane forms around newly separated chromatids Note that each new nucleus formed has ½ the amount of DNA as the original cell. These cells are haploid cells.

Nondisjunction

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Click on image to play video.

How can siblings look alike but not exactly the same if they come from the same parents?

Crossing over The chromosomes during prophase I undergo crossing over, where parts of the homologues randomly switch places.

Importance of crossing over The gene combinations that a person gets from his or her parents will be different, to varying degrees, than the combination a sibling may get.

More sibling similarities

Variations on Mendel Linked genes: Mendel concluded that traits are assorted independently, but some traits are linked. This means that two genes are almost always inherited together (ex: red hair, green eyes).

Fruit fly chromosome #2 Exact location on chromosomes Chromosome 2 For example, a fly with reddish-orange eyes and miniature wings, were almost always linked, inherited, together.