Mendel Genetics

Our Main Man Mendel!• Gregor Mendel father of

genetics; discovered trends in heredity 30 years before other scientists!

• Prior to Mendel what did people believe about inheritance?– Blending Theory all genetic

traits are equally mixed in offspring

– If this were true what would disappear?• Outliers (really tall, short,

etc… members of a population)

Mendel’s Work• Secret to Mendel’s discovery

is his detailed record keeping and attention to the controlled experiment

• What was his model species?– Garden peas

• What did this species work so well?– Easily grown; has set traits

(characteristics) that are easily to observe; character differences mostly in twos (white flowers vs. purple; tall vs. short)

7 Traits of Mendel’s Experiments

• If you are going to observe trait inheritance, what must you be pretty sure of?– The genetic make up of

your subjects• Pollination- transfer of pollen

(male gametes) to the female reproductive organ in plants– To make pure-breeds for

each trait• Cross-pollination- using pollen

from a plant with different traits to test the trait inheritance patten

Mendel’s Monohybrid Crosses• Wanted to produce specific

hybrids of plant with different traits – Tall X Short– Purple Flowers X White

Flowers– Green Seeds X Yellow Seeds

• Started with P1

– Parents• Offspring were call F1

– Filial (son/daughter)– 1st generation

• F1 X F1 = F2

– 2nd generation

3

1

Controlling Heredity• Each organism has two factors that control

each of its traits.

– Genes exist in alternative forms. We call

these different gene forms alleles

– Alleles are located in the same locus

(area) of homologous chromosomes

(chromosome sets)

– Allele that determines the appearance

(phenotype) is dominate

– Other allele is recessive • Genotypes :

– Homozygous dominate AA– Homozygous recessive aa– Heterozygous Aa

Alleles are RR, YY, PP, TT or Rr, Yy, Pp, Tt…

Alleles are rr, yy, pp, tt, etc…

Mendel’s Three Theories• From the first cross all F1 flowers are

purple• From F1 cross 3 purple: 1 white flower • Theories1) The parents must carry a pair of

factors (alleles) the control the inheritance of each trait– P purple flowers– p white flowers

2) If you have one of each allele, one will be dominate and one recessive– Pp purple flowers

3) Pairs of alleles are segregated into different gametes– Law of Segregation

• Gamete has either P or p

Punnett Squares• Reginald Punnett; English

biologist

• Monohybrid cross

following the crossing of

parents while focusing on 1

trait

• A cross with 1 homozygous

dominate and 1

homozygous recessive

makes what?

• Only heterozygous

offspring

• A cross between 2

heterozygous produce

what?

• ¼ TT, ¼ tt, and ½ Tt

Monohybrid Crosses • Punnett Square method for organizing

alleles during breeding that uses statics – All Statics probabilities are between 0-1

• 0impossible• 1 completely possible

• Product Rule events where both A and B will occur– Chance of heads vs. tails 0.5– Chance of heads twice 0.5x0.5= 0.25

• Sum Rule when two or more possible ways exist to get the same outcome– Chance of getting head and tail in 2

tosses 0.5x0.5=0.25 – Two ways (head and tail or tail and

head) 0.25+0.25= 0.5

Practice Crosses1) What are the possible genotype ratios

for a cross between homozygous dominate (purple) and homozygous recessive (white) garden pea plants? What is their phenotypes?– 1.0 chance of heterozygous (Pp)– All purple flowers

2) What are all possible genotype ratios for a green seed X yellow seed? Green is dominate.– 1.0 chance heterozygous (Pp)– 0.5 chance heterozygous (Pp) and 0.5 homo

recessive (pp)– All green seeds; half green and half yellow

seeds

P P

p Pp Pp

p Pp Pp

G G

g Gg Gg

g Gg Gg

G g

g Gg gg

g Gg gg

Genetic Terminology • What is a genotype?

– Allele pairing in genes• What is a phenotype?

– Physical appearance of alleles• Word for AA genotype?

– Homozygous dominant• Word for aa genotype?

– Homozygous recessive• Word for Aa genotype?

– Heterozygous• What do you call a cross between

two subjects that follows the inheritance of one trait?– Monohybrid Cross

Dihybrid Cross• Cross following two traits

with both subjects being heterozygous for each trait (AaBb X AaBb)

• What ratio does a dihybrid cross always produce?– 9:3:3:1– 9 AB; 3 Ab; 3 aB; 1 ab

• What does this ratio tells us about alleles for A and B?– They each sort randomly into

gametes (not linked)– Principle of Independent

Assortment

Practice Crosses• What is ratio of the

phenotypes of a RRYy x Rryy?

• R round seed• r wrinkled seed• Y yellow seed • y green seed

– 1.0 round seed; 0.5 yellow and 0.5 green seed

• What phenotype will not be present after a Rryy x rryy cross?– Yellow seeds

RY RY Ry Ry

Ry RRYy RRYy RRyy RRyy

Ry RRYy RRYy RRyy Rryy

ry RrYy RrYy Rryy Rryy

ry RrYy RrYy Rryy Rryy

Ry ry Ry ry

ry Rryy rryy Rryy rryy

ry Rryy rryy Rryy rryy

ry Rryy rryy Rryy rryy

ry Rryy rryy Rryy rryy

Chromosome Theory of Inheritance• Sum up what we have learned from

Mendel:1) Chromosomes occur in pairs in

sexually reproducing, diploid organisms. Alleles for each gene are on these chromosomes

2) The chromosomes of each pair are separated and delivered to different gametes. This also separates alleles of genes

3) The separation of any pair of chromosomes in meiosis and gamete formation is independent

4) One member of each chromosome pair after fertilization comes from the male and female parents

Alternative Inheritance Patterns1) Incomplete dominance– Phenotype for heterozygous

genotype is a blend of both homogenous traits

– Recessive trait is not completely blocked because dominate trait is incomplete

– CRCR x CWCW give what type of offspring ratios?• 100% pink (heterozygous)

– What about F2?• 1 Red: 2 Pink: 1 White

Alternative Inheritance Patterns2) Codominance– Both alleles are expressed

equally and produce equal effects in heterozygotes

– MN blood groups• People either have M form

glycoproteins or L form glycoproteins• If you get both alleles you

have both glycoproteins

– Same inheritance patterns as incomplete dominance

Alternative Inheritance Patterns3) Multiple Allele Inheritance– Several types of same allele

will code for different phenotypes

• Human Blood-Types– Allele I-> IA, IB, or i IAIA IBIB IAIB ii

Alternative Inheritance Patterns4) Epistasis gene interaction; genes on one allele hide the phenotypes of genes on another allele– Common in skin tones and

animal colors• B and C allele produce dark

pigments while b and C produce less/no pigment

• B and C needed to make black• C needed to make brown if b• No C means white regardless

of B or b

Alternative Inheritance Patterns5) Polygenic Inheritance– Several genes contribute to

the same trait (quantitative trait); skin color, height, weight, etc…

– Typically shows bell-curve distribution in a population

Sex Chromosomes• Sex determination

– chromosomes, which

determine the sex of an

individual, are called sex

chromosomes

– 22 other chromosomes

are called autosomal

chromosomes

Sex-Linked Inheritance• Drosophila eye color:• “Wild type” Red, w+w+• Mutation White, ww• R dominate over r• What is the F1 genotype?• heterozygous; w+ w• What is the F2 ratio?• 3:1 1 w+w+, 2 w+ w, 1 ww• F2 only produces males with white eyes• Sex-linked inheritance

Sex-linked Inheritance• Y Chromosome– Sex-determining genes; SRY gene

makes females into males while an embryo

– Maybe fading from existence; may be getting smaller

– XY heterogametic • X Chromosome– Mostly codes for non-sex related traits

(ex. Color vision)– XX homogametic

• Colorblindness• Hemophilia– Low platelet number makes it difficult

to stop bleeding– Influences a revolution in Russia

Too Many Xs!• Why do females need two Xs?

– They Don’t! Two X chromosomes would mean double the genetic material necessary

• What does the body do with the X chromosome?– It randomly shuts one X down– Creates a Barr body dense mass of

inactive chromatin– They are copied and passed on in

mitosis but are never used for proteins• How can this show us X-recessive

traits?– Dominate X might be randomly

deactivated so the X recessive is randomly present in cells

– Female calico cats have a mix of orange and black fur but males are always black or orange

Environmental Effects

• Environmental Factors:– Temperature, nutrition, light, etc… can effect the

phenotype of an individual by changing the expression of the genes

– Arctic animals are influence by cold

• Internal Factors:– Hormones regulate gene expression so a genotype may

not express the same phenotype it different individuals

– Peacock color is not sex-linked, but the gene that regulates the color gene is