The life and work of Gregor Mendel

Over seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful? Pea plants grow quickly. Pea plants are available in pure-breeding (homozygous) strains. Many pea plant characteristics show discontinuous variation; they are either one form or another, with no intermediates. This means that it is easy to distinguish one trait of pea plant from another pea plant with a different trait.

Genetic Terminology  Trait - any characteristic that can be passed from parent to offspring  Heredity - passing of traits from parent to offspring  Genetics - study of heredity 4

Types of Genetic Crosses  Monohybrid cross - cross involving a single trait e.g. flower color  Dihybrid cross - cross involving two traits e.g. flower color & plant height 5

Ways to describe Genes  Alleles - two forms of a gene (dominant & recessive)  Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)  Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 6

Members of the same family often look similar. If the son and daughter have children of their own one day, will they also look like their parents? Which parent do these children look more like? Humans, like all organisms, inherit characteristics from their parents. How are characteristics passed on? Why do members of the same family look similar?

Chromosomes in a homologous pair contain the same type of genes that code for the same characteristics, such as eye color. Each different version of a gene is called an allele. Each chromosome in the pair, however, may have a different version of the gene. For example, the version of a gene on one chromosome may code for brown eyes, whereas the version of the gene on the other chromosome may code for blue eyes. allele for brown eyes allele for blue eyes

If the alleles for a characteristic in a homologous pair are the same, the organism is said to be homozygous for that characteristic. What color eyes will these homozygous pairs of alleles produce? allele for brown eyes allele for blue eyes

The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive. If the alleles for a characteristic in a homologous pair are different, the organism is said to be heterozygous for that characteristic. What color eyes will this heterozygous pair of alleles produce? allele for brown eyes allele for blue eyes ?

More Terminology  Genotype - gene combination for a trait (e.g. RR, Rr, rr)  Phenotype - the physical feature resulting from a genotype (e.g. red, white) 12

Genotypes  Homozygous genotype – When the two alleles are same (2 dominant or 2 recessive genes) e.g. TT or tt; also called pure  Homozygous genotype – When the two alleles are same (2 dominant or 2 recessive genes) e.g. TT or tt; also called pure  Heterozygous genotype – When the 2 alleles are different- one dominant & one recessive allele (e.g. Tt); also called hybrid 13

Phenotype –Physical characteristics

Genotype Phenotype –Physical characteristics Genotype –Genes we inherit from our parents

Phenotype –Facial structure Notice the similarities:

Phenotype –Facial structure –Eyes Notice the similarities:

Phenotype –Facial structure –Eyes –Smile Notice the similarities:

Phenotype –Facial structure –Eyes –Smile –Ears Notice the similarities:

Phenotype –Facial structure –Eyes –Smile –Ears –Nose Notice the similarities:

Phenotype –Facial structure –Eyes –Smile –Ears –Nose –Neck Notice the similarities:

Dominant alleles are always expressed in a cell’s phenotype. Only one copy of the dominant allele needs to be inherited in order for it to be expressed. Dominant alleles (e.g. brown eyes) are represented by an upper case letter (e.g. ‘B’). The phenotype for a particular characteristic depends on which allele is dominant and which allele is recessive. Recessive alleles are only expressed in a cell’s phenotype if two copies of it are present. If only one copy is present, its effect is ‘masked’ by the dominant allele. Recessive alleles (e.g. blue eyes) are represented by a lower case letter (e.g. ‘b’).

The allele for brown eyes is dominant over the allele for blue eyes. The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes. allele for brown eyes allele for blue eyes So, what color will the eyes be of an individual who is heterozygous for eye color?

Inheritance terms

There are two alleles controlling pea shape. This means there are three possible genotypes that the F2 generation of plants could inherit, leading to two possible phenotypes. SS ww Sw smooth wrinkly smooth Genotype heterozygous homozygous dominant homozygous recessive Phenotype Mendel carried out experiments investigating just a single characteristic, which is called a monohybrid cross. The likelihood of a trait being produced during a monohybrid cross can be mapped out using a Punnett Square.

Predicting Inheritance To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes. UPPERCASE letters are used to represent dominant genes. lowercase letters are used to represent recessive genes.

Predicting Inheritance For example: T = represents the gene for TALL in pea plants t = represents the gene for short in pea plants So: TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant. Remember there are two genes for every trait! One from each parent.

Predicting Inheritance Let’s cross a totally dominant tall plant (TT) with a short plant (tt). Each plant will give only one of its’ two genes to the offspring or F 1 generation. TT x tt TTtt

Punnett Square Used to help solve genetics problems 29

Punnett Squares TtTtTt The genes from one parent go here. The genes from the other parent go here.

Punnett Squares TT tTtTtTt t

Punnett Squares TT tTtTtTt tTtTt

Punnett Squares TT tTtTtTtTt tTtTtTtTt

TT tTt tTtTtTtTt

Punnett Squares TT tTt t

Punnett Squares TT tTt t F 1 generation

Interpret the Results using Probability Likelihood that a specific event will occur Expressed as: –Decimal, percentage, ratio, or fraction

Interpreting the Results The genotype for all the offspring is Tt. The genotype ratio is: Tt - 4/4 The phenotype for all the offspring is tall. The phenotype ratio is: tall - 4/4

Punnett Squares Tt T?? t Your Turn!!

Punnett Squares Tt TTTTt t tt F 2 generation

Punnett Squares Tt TTTTt t tt Next, give the genotype and phenotype ratios of the offspring (F 2 generation).

Punnett Squares Tt TTTTt t tt Genotype ratio: TT - 1

Punnett Squares Tt TTTTt t tt Genotype ratio: TT - 1, Tt - 2

Punnett Squares Tt TTTTt t tt Genotype ratio: TT - 1, Tt - 2, tt - 1

Punnett Squares Tt TTTTt t tt Genotype ratio: TT - 1, Tt - 2, tt - 1 Phenotype ratio: Tall - 3

Punnett Squares Tt TTTTt t tt Genotype ratio: TT - 1, Tt - 2, tt - 1 Phenotype ratio: Tall - 3, short - 1

Monohybrid Cross Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr (Parent P 1 ) 47 R R rr Rr Genotype:Rr Genotype: Rr PhenotypeRound Phenotype: Round Genotypic Ratio: Genotypic Ratio: 100% Rr Phenotypic Ratio:100% round Offspring = F 1

F 1 Monohybrid Cross Trait: Seed Shape Alleles: R – Roundr – Wrinkled Cross: Round seeds x Round seeds Rr x Rr 48 R r rR RR rrRr Genotype:RR, Rr, rr Genotype: RR, Rr, rr PhenotypeRound & wrinkled Phenotype: Round & wrinkled G.Ratio:RR 25%, Rr 50%, rr 25% G.Ratio: RR 25%, Rr 50%, rr 25% P.Ratio: Round 75% & wrinkled 25%

50 Test Crosses When an organism shows a dominant phenotype how do you know if it is homozygous or heterozygous? Mendel performed test crosses – he crossed the unknown with a plant that was homozygous recessive. Rrrr Rrrr Rr r r Rr r r R R If the unknown is homozygous all the F1 show the dominant phenotype. If it is heterozygous the recessive and dominant phenotype are equally expressed

Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same 51

Dihybrid Cross Cross between individuals that involves two pairs of contrasting traits

Mendel’s Laws Law of Segregation Law of Dominance Law of Independent Assortment 53

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. 54

Mendels’ “Law” of Segregation Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait.

Law of Dominance 56 In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. Example: RR x rr yields all Rr (round seeds)

Law of Independent Assortment Alleles for different traits on different chromosomes are distributed to the sex cells independently of one another. 57

Independent Assortment Example Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). All of the F 1 offspring produced round yellow peas (RrYy). 58 The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).

Independent Assortment Example Mendel crossed the heterozygous F 1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F 2 generation. RrYy × RrYy 59 The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F 2 generation. In Mendel’s experiment, the F 2 generation produced the following: some seeds that were round and yellow some seeds that were wrinkled and green some seeds that were round and green some seeds that were wrinkled and yellow

Independent Assortment Example The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. Genes that segregate independently do not influence each other's inheritance. 60 Tt B b The diploid cell TtBb can make 4 possible gametes: TBTb tBtb The segregation of the Tt alleles is independent of the Bb alleles