Genetics Heredity – the passing of traits from parent to offspring Genetics- the study of heredity Part I: Mendel and the Gene Idea

I. Gregor Mendel He was a monk, gardener, mathematician 1st to apply statistics to selective breeding Published work on pea plant inheritance patterns in the 1860’s. (nothing known about the cell for inheritance…)

Why was the pea a good choice? 1) Cheap and available 2) Produce offspring quickly/short generation time 3) Characteristics are determined on 1 gene with only 2 alleles. Ex. Flower color: purple or white 4) No trait blending 5) Can control breeding. (Easy access to male and female sex parts)

Mendel’s Questions Mendel crossed (bred) two different plants to discover what traits the offspring would show. Ex: Will a purple flower plant crossed with a purple flower plant produce all purple offspring?

II. Basic genetic concepts A. Mendel studied 7 different characters in peas: Ex) Height: tall vs. dwarf Seed shape: smooth vs. wrinkled

Characters – inherited features of an organism. Traits – variations of a character. Ex) character: fur color possible traits: brown, black, red

Review of chromosomes A. Each = one DNA molecule B. Gene – DNA sequence found on a particular chromosome, that codes for a particular trait. C. Each may have thousands of genes for a particular set of genes Ex. Human chromosome 11 has the genes for making the enzymes hemoglobin, catalase, and insulin (and thousands of other genes)

Alleles are possible forms of the same gene. Genes express alleles Alleles are possible forms of the same gene. Eye color brown blue

More about alleles: Dominant allele – the “stronger” allele. In a heterozygous combination the dominant allele will be expressed. Symbol is a capital letter Ex) trait: pea plant height tall is dominant, tall = T Recessive allele – “weaker” allele. In the heterozygous individual these are hidden, and the dominant form of the trait will be expressed. Symbol is a lowercase letter dwarf is recessive, dwarf = t

Allele combinations for genes HOMOZYGOUS – both gene are the same. Ex) Seed coat trait: wrinkled seed allele and wrinkled seed allele HETEROZYGOUS – genes are different Ex) seed coat trait wrinkled seed allele and round seed allele f

Genotype: the alleles an individual has Phenotype: the way those alleles are expressed Ex) pea plant height Possible genotypes: T T, T t, or t t Possible phenotypes: tall or dwarf Possible combinations: Homozygous dominant = TT, tall Homozygous recessive = tt, dwarf Heterozygous = Tt, tall

Example of an individual’s allele combinations on three gene loci

Mendel’s Experiments True-breeder – an individual that always produces offspring with the same expression for a given trait. Ex) purple flower plant always produces purple flower offspring P1 – the parent generation. Two true breeders with different traits for the same character are crossed Ex) true breeding purple flower pea X true breeding white flower pea F1 – first filial generation. The offspring (progeny) of the P1 F2 – second filial generation. The offspring of two individuals from the F1 generation.

Experiment Results

C. Mendel’s Conclusions Mendel’s Rules of Inheritance – generalizations made by Mendel Law of Segregation Law of Independent Assortment

Mendel’s Rules of Inheritance Different versions of genes account for variety in organisms. 2) For each character, an organism inherits two alleles, one from each parent. 3) If two alleles differ, then one (dominant) is fully expressed, and the other (recessive) has no effect.

Mendel’s Laws of Inheritance Law of Segregation - The two alleles a parent has for each character segregate during gamete production. Ex. Heterozygote pea for flower color Pp : half of gametes get P, other half get p Law of Independent Assortment – for each gene the alleles separate independently of alleles for other genes Ex) heterozygote for flower color (Pp) and seed color (Gg) some gametes will get PG, some will get Pg, pG, or pg, in a ¼ ratio Refers to two or more genes/characters

A Test Cross WHY? To determine whether an organism that has the dominant trait is homozygous dominant or heterozygous. HOW? Cross with a homozygous recessive organism. Does the recessive phenotype show up? How to read the results? 100% dominant offspring = homozygous dominant parent ANY recessive offspring = heterozygous parent.

Patterns of Inheritance NOT revealed by Mendel’s studies Incomplete dominance Codominance Multiple alleles Polygenic traits

Incomplete dominance In heterozygotes both alleles are expressed so the trait blends. Example: flower color in snapdragons allele F R = red allele F W = white In heterozygotes (F R F W) = pink

Codominance In heterozygotes both alleles are expressed in separate distinguishable ways Example: Roan horse RR – there are red hairs rr – there are white hairs Rr – some hairs are white and some are red

Multiple alleles There are more than two possible alleles for a character. Example: human ABO blood group 3 alleles A, B, and O (allele = O) O is recessive, A and B are codominant 4 possible phenotypes Type A blood (genotypes: AA or AO) Type B blood (BB or BO) Type O blood (OO) Type AB blood (AB)

Human Blood Types

Polygenic inheritance -many genes affect one phenotype. Ex) human skin pigmentation

Sex-linked genes – genes that are on the X chromosome show unique inheritance patterns The Y chromosome (ONLY male gender). The X chromosome has other genes on it. Sex-linked traits can pass from mom to sons OR daughters can pass from the dad to ONLY daughters

Mendelian Inheritance in humans: - from dominanat/recessive alleles on one gene. Examples: Widow’s peak Attached or free earlobes Recessive disorders*: Ex.s) cystic fibrosis, Tay-Sachs disease, sickle-cell disease Dominant disorders*: Ex.) Huntington’s *Determined with pedigree analysis, genetic testing and counseling