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 F 1 – first offspring generation – All had purple flowers – These plants were allowed to self-fertilize

F 2 – 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 F 1 Pp - purple flowers

F 1 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 F 1 all round F 2 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 F1all 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

RYRyrYry 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 RW RRR - redRW - pink W 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

I A i x I B i IAIA i I B I A I B - ABI B i - B iI A i - Ai 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

X C X c x X C Y XCXC X c XCXC Y

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