Genetics

How are traits passed from parents to offspring?

Gregor Mendel

• Austrian monk• Experimented with pea plants– Produce lots of offspring– Readily observable traits– Mating is easily controlled– Pure-breeding strains were available

• Studied several traits, but only one at a time

• Phenotype – physical appearance of an organism

• Tall or short• Round or wrinkled• Yellow or green seed

Parental generation cross

• Parents – purple flowers and white flowers – Both were pure breeding

• F1 – first offspring generation– All had purple flowers– These plants were allowed to self-fertilize

• F2 – second offspring generation– 3 purple-flowered plants– 1 white-flowered plant

• Where was the white-flowered trait in the parents (all purple)?

• Why do the crosses produce a 3:1 ratio of purple to white?

• Mendel concluded that the white trait must be hidden in the purple flowers

• The trait that is hidden or masked is said to be recessive

• The trait that is expressed is said to be dominant

• Purple is dominant over white

Law of Segregation

• The 3:1 ratio suggested to Mendel that each organism carried two factors (genes) for the trait flower color

• These factors separate (segregate) during the formation of sex cells (pollen and eggs) so that each sex cell only has one factor (gene) for a trait

Genotype

• Genetic make-up of an organism• Allele – forms of a gene– Ex- flower color alleles are purple and white

• Homozygous –having two of the same allele for a gene

• Heterozygous- having one dominant and one recessive allele

Representing the genotype

• Dominant allele is represented by capital letter

• Recessive allele is represented lower-case letter

• Ex – Flower color– P – purple– p - white

Parent cross

• Pure purple X pure white• PP pp• Sex cells P p

• F1 Pp - purple flowers

F1 self fertilizes

• Pp x Pp

• Sex cells P p P p

• Offspring PP Pp Pp pp

• Sex cells combine randomly

• Phenotype ratio – 3 purple : 1 white• Genotype ratio – 1 PP : 2 Pp : 1 pp

Punnett square

• Write the sex cells from one individual along the left-hand side of the square

• Write the sex cells from the second individual along the top

• Combine row and column to produce the offspring genotypes

Dihybrid crosses

• Monohybrid crosses follow one trait at a time• Dihybrid crosses follow two traits

• P Round seeds X wrinkled seeds

• F1 all round

• F2 3 Round : 1 wrinkled• Conclusions : round is dominant over wrinkled

• P Yellow seed x green seed

• F1 yellow seed

• F2 3 Yellow : 1 green• Conclusion: Yellow is dominant over green

• P Round-Yellow x wrinkled-green

• F1 all round-yellow

• F2 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

• Note: 12 round : 4 wrinkled• 12 yellow : 4 green

The F2 phenotype ratios are unchanged

How did Mendel explain these observations?

Law of independent assortment

• The inheritance of one trait has no effect on the inheritance of a second trait

• Ex- the shape of a seed does not affect its color and vice versa

• When sex cells are formed, a cell receives one allele for each trait being studied

• F2 RrYy x RrYy

• Sex cells

RY Ry rY ry

RY

Ry

rY

ry

Results

• 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green

• 1 RRYY: 2 RRYy: 1 RRyy• 2 RrYY : 4 RrYy : 2 Rryy• 1 rrYY : 2 rr Yy : 1 rryy

Non-Mendelian Patterns of Inheritance

Incomplete dominance

• Two or more alleles influence the phenotype producing an individual that is intermediate between the two alleles

• Ex. Four-o-clocks• Red Flowers X White Flowers

Pink Flowers

Red X White

• R – red• W – white

• RR x WW• Sex cells R W• F1 RW pink

Pink x Pink

• RW x RW• Sex cells R W R W

R W

R RR - red RW - pink

W RW - pink WW - white

• Phenotype ratio– 1 red: 2 pink: 1 white

• Genotype ratio– 1 RR : 2 RW : 1 WW

Codominance

• Both alleles are expressed• There is no blending

Example – ABO blood system

• I A - blood cell carries polysaccharide A• I B - blood cell carries polysaccharide B• i – neither A nor B is present

IA i x IB i

IA i

I B IA IB - AB IB i - B

i IA i - A i i - O

Multiple allelic inheritance

• More than two alleles for a trait• Example – ABO blood system

Sex-linked traits

• Traits such as hemophilia and red-green color blindness occur more often in males than females

• Traits are found on the sex chromosome (X)• Females - XX• Males - XY– Males only receive one allele for these traits

XC Xc x XC Y

XC Xc

XC

Y

Polygenic Inheritance

• Many genes influence a trait• Examples – height, eye color, skin color• Graded variation in the trait