Chapter 12: Mendel and Heredity

Heredity – The passing of traits from parents to offspring Genetics – The study of heredity

Gregor Mendel Austrian monk Bred pea plants developed laws of heredity

He cross-pollinated plants mms:// /Video9/mendelslaw.asf

He bred plants to be pure for certain traits Ex: Tall parent  tall offspring Short parent  short offspring Then he cross-bred plants with opposite traits Tall x short Round x wrinkled Yellow x green

Parents - P generation – tall x short Offspring – F 1 generation (1 st filial generation) All offspring were tall (short trait disappeared) Allowed F 1 generation to self-pollinate F 2 generation – 75% tall to 25% short (short trait reappeared) Repeated many times – always same ratios for each generation (see results slide #2)

Mendel’s Conclusions: There are 2 factors for every trait (today we know these factors to be genes – 1 from mother, 1 from father) One of these factors can be dominant over the other (the recessive trait) This is known as the Law of Dominance

Alleles – each version of a gene Ex: tall and short height curly and straight hair brown and blue eyes

What is the relationship between chromosomes, DNA, genes, and alleles? Chromosomes contain DNA and protein DNA contains genes – each gene always has the same position on a chromosome (locus) Alleles are different forms of the same gene (right vs. left-handed, blue vs. brown eyes)

Widow’s peak

When an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles –The Law of Segregation Each gamete only has 1 factor from each pair (haploid) Fertilization gives each new individual 2 factors again (diploid)

Mendel then crossed pure plants that differed in 2 traits Ex: yellow, round peas crossed with green, wrinkled peas F 1 generation always showed dominant traits F 2 generation had the following results: (see next slide)

F 2 :9 yellow, round 3 yellow, wrinkled 3 green, round 1 green, wrinkled

Based on these results, Mendel concluded that during gamete formation, the alleles of each gene segregate independently – The Law of Independent Assortment Ex: Below, hairline and finger length are not dependent on each other

Mendel’s Law of Independent Assortment would not hold true if the genes for two traits are located on the same chromosome – linked genes

Genotype and Phenotype Genotype refers to the set of alleles that an individual has for a character; can be represented by two letters Homozygous - both alleles are the same Homozygous dominant - WW Homozygous recessive - ww Heterozygous – alleles are different - Ww

Phenotype refers to the trait that results from a set of alleles. Both WW and Ww result in widow’s peak, the dominant trait ww will result in no widow’s peak, the recessive trait

Monohybrid Crosses Considers only one trait. Punnett square – chart that shows all of the genotypes that could result from a given cross Ratio shows # of offspring with dominant vs. recessive trait

Probability The likelihood that a specific event will occur. Expressed as fraction or percentage Ex: (1/4) or 25% The probability that two or more independent events will occur together is the product of their chances occurring separately Ex: odds of having a boy = ½ Odds of having 2 boys = (1/2) x (1/2) = (1/4)

The chance of widow’s peak: WW or Ww = 75% or ¾ Chance of a continuous hairline: ww = 25% or 1/4 Odds of having 3 children with a continuous hairline: (1/4) x (1/4) x (1/4) = (1/64)

Dihybrid Cross Two traits are considered Genotypes of the parents require four letters (two for each trait).

Multiple Allelic Traits More than two alternative alleles exist for a particular trait. Ex: blood type – A, B, and O are the 3 alleles Each individual inherits only two alleles for these genes.

Codominance - both alleles for the same gene are fully expressed in a heterozygote Ex: Blood type – AB blood Incomplete dominance – An offspring has a phenotype that is intermediate between the traits of its parents. Ex: Curly, wavy, or straight hair in Caucasians

ABO Blood Types How your book shows blood type: Blood type (phenotype) Genotype AI A I A or I A i BI B I B or I B i ABIAIBIAIB Oii

Blood type (phenotype) Genotype AAA or AO BBB or BO AB OOO How your teacher shows blood type:

Blood typeDonor forRecipient from AA, ABA, O BB, ABB, O AB A, B, AB, O O O

Inheritance of blood type

Incomplete dominance

Other examples of incomplete dominance: Plants called four o’clocks RR – red RR’ – pink R’R’ – white So a cross between two pink plants produces 1 red, 2 pink, and 1 white plant RRRR’ R’R’ RR’ R

Another example includes Sickle cell disease in humans Hb A represents normal hemoglobin; and Hb S represents the sickled condition –Hb A Hb A – normal –Hb S Hb S – sickle-cell disease –Hb A Hb S - have the intermediate condition called sickle-cell trait. Heterozygotes have an advantage in malaria- infested Africa because the pathogen for malaria cannot exist in their blood cells.

Sex determination: Female – XX Male – XY Always 50% chance of having a boy or a girl Male determines gender of baby XX XY XX X Y

Sex-Linked Traits Traits controlled by genes on the X or Y chromosomes X-linked or Y-linked Most X-linked traits are recessive, so a female would have to have two recessive genes to express the trait; a male would only need one. Y-linked traits are only passed from father to son

Examples of X-linked traits include Color blindness, Hemophilia, & Muscular Dystrophy PhenotypeGentoype Normal femaleXBXBXBXB Carrier femaleXBXbXBXb Affected female XbXbXbXb Normal maleXBYXBY Affected maleXbYXbY

Cross involving an X-linked allele

Sex Influenced Traits Sex-influenced traits are autosomal traits that are influenced by sex. If a male has one recessive allele, he will show that trait, but it will take two recessive for the female to show that same trait. One such gene is baldness. BB  normal male & female Bb  bald male; normal female bb  bald male; bald female

Pedigree Charts Constructed to show the pattern of inheritance of a characteristic within a family. The particular pattern indicates the manner in which a characteristic is inherited (suggests X-linked, dominant, etc.)

Normal female Carrier female Affected female Normal male Affected male Symbols used: Male can be a carrier for an autosomal trait.

Autosomal recessive pedigree chart

Autosomal dominant pedigree chart