Introduction to Genetics

Sexual Reproduction:

– Zygote is a product of both parents

– The chromosomes of each gamete (egg and sperm) bring hereditary material from each parent

– Offspring is both similar and different from both parents

Phenotypes and Genotypes

• Phenotype: the way an organism looks or appears

• Genotype: the actual gene combination an organism has (may not be visually apparent)

Development of Genetic Theories

Gregor Mendel (Mid 1800s)

• Studied pea plants.

• How were traits passed on from one generation to the next

• Discovered basic laws of genetics

• Crossed plants that were purebred for different traits– Purebred: organisms that produced

organisms identical to themselves

• Studied one trait at a time

Monohybrid Cross: – Parents differ by a single trait– Offspring of this type of cross are called hybrids

• Ex: Mendel crossed tall and short pea plants

Mendel’s First Experiment

• Parent Generation (P1)

• 1st generation (F1): – All tall offspring– What happened to the

short?

• 2nd generation (F2): – ¾ tall and ¼ short– The short trait

reappeared!!

• He repeated his tests for 7 other traits in pea plants and got same result.

– One trait disappears in F1 generation– But reappears in ¼ of F2 generation

WHY???

• Bill Nye:

• Greatest Discoveries: Law of Inheritance

• http://www.youtube.com/watch?v=0vAAf4g5iF8&safe=active

• Online Activity

• http://www.hhmi.org/coolscience/resources/SPT--FullRecord.php?ResourceId=73

Mendel’s Rules for Inheritance

Rule of Unit “Factors”

Characteristics are determined by factors that are passed from one generation to the next

– Each organism has 2 “factors” for each trait– These “factors” can exist in alternate forms

We now call these factors

genes

These genes can exist in

different forms or alleles

Organisms inherit one allele

from each parent

(on homologous chromosomes)

Mendel’s Rules for Inheritance

Rule of Dominance• Some alleles are dominant and some recessive

• EX: – TT and tt pea plants are crossed – All offspring are:

• Genotypically Tt (hybrid) • Phenotypically Tall

– Trait that phenotypically appears is dominant (Tall)– Trait that disappears is recessive (short)

Genotype Phenotype

TT Tall

Tt Tall

tt Short

Dominance in Human Traits

Dominant Recessive

Freckles No freckles

Curly hair Straight hairHair on middle joint of finger No hair on middle joint of finger

Widows peak No widows peak

Unattached earlobe Attached earlobe

Curl tongue Can’t curl tongue

Mendel’s Rules for Inheritance

Law of Segregation How are different forms of agene distributed to offspring?

During gamete formation(meiosis), alleles for eachgene segregate or separateindependently from each other

Each gamete only carries oneallele for each gene.

• Homozygous: Individuals that have two of the same alleles for a trait (ex: BB or bb)

• Heterozygous: “hybrid” individuals that have two different alleles for the same trait. (ex: Bb)

Punnet Squares

• Illustrates the law of segregation of traits

• Shows all possible combination of gametes

• Way of finding expected proportions of traits in offspring– (Need a large sample size to really get

expected ratio)

• Monohybrid Cross: • Ex: Parents differ by single trait for height

– TT x tt– Tt x Tt– Tt x tt

– Are the parents in each cross homozygous/heterozygous?

– What are the phenotypes for each parent?

– Let’s cross

– What are the outcomes?

– What are the genotypes and phenotypes of the offspring?

• Dihybrid Cross:– Cross involving two different traits

– Ex: Round yellow seeds x wrinkled green seeds– Round = R, Wrinkled = r– Yellow = Y, Green = y

– Let’s try a cross! RRYY x rryy

– P1 Generation: RRYY x rryy

– F1 Generation: All round yellow (RrYy)

– 100% heterozygous hybrids for both traits– Phenotype is 100% yellow round– Round and yellow traits must be dominant!

– F2 Generation: – Cross “dihybrids” from the first generation

• RrYy x RrYy

• Create a Punnet Square:

– What goes at top of each column and row?

– What are the possible combination of these genes in the gametes?

– Phenotype Outcome: 9 : 3 : 3 : 1Yellow Round Green Round Yellow Wrinkled Green Wrinkled

9 3 3 1

• Do you get a 9:3:3:1 phenotype ratio?

Law of Independent AssortmentAlleles for different traits are inherited independently of

each other.

During gamete formation (meiosis) chromosomes are

separating independent of each other

• Ex: – Round/wrinkled and yellow/green traits are inherited

independantly of each other

• Note: – Only true if the genes for traits are located on different

chromosomes

Mendelian Genetics SummaryLaw of Unit Factors:

– Traits have 2 factors (or alleles) that can be inherited from parentsLaw of Dominance:

– One factor is dominant and the other recessiveLaw of Segregation:

– During gamete formation the factors segregate randomly into eggs and sperm and then recombine in offspring

Law of Independent Assortment: – Different traits sort themselves independently of each other

• Phenotype (how it looks) vs. Genotype (what genes/alleles it has)• Homozygous (BB or bb) vs. Heterozygous (Bb)• Monohybrid Cross (hybrid for one trait) get 3:1 phenotype ratio• Dihybrid Cross (hybrids for 2 traits) get 9:3:3:1 phenotype ratio• Punnet Square: know how to set them up!