Dihybrid Crosses and Law of Independent Assortment

Monohybrid cross Mendel’s experiments followed the inheritance of single characters – E.g. flower color, seed color, etc. – monohybrid crosses

Law of Segregation Review – During gamete formation, alleles segregate (separate) only one of two allelles for a gene is put into a gamete. – At fertilization, offspring receive one allele for each gene from each parent. – alleles are on different chromosomes!

Dihybrid cross Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and seed shape – dihybrid crosses Mendel was working out many of the genetic rules!

Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P 100% F 1 generation (hybrids) yellow, round peas Y = yellow R = round y = green r = wrinkled self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas F 2 generation YyRr

What’s going on here? If alleles (different characteristics) are on different chromosomes… – Are alleles transmitted from parents to offspring together or independently? – Will the Y and R alleles always stay together, generation after generation? YyRr YRyr YyRr YryRYRyr Is it this?Or this? Which system explains the data? Y and R alleles transmitted independently Y and R alleles transmitted together

9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled Is this the way it works? YyRr YRyr YR yr x YyRr YryRYR yr YyRr YRyr or YYRRYyRr yyrr Well, that’s NOT right! What was actually observed

Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRR x YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled YyRr YryRYR yr YyRr YRyr or BINGO!

Mendel’s 2 nd law of heredity round wrinkled Law of independent assortment – Each pair of alleles (different characteristics) segregate independently non-homologous chromosomes align independently only true if alleles (different characteristics) are on separate chromosomes (e.g. gene for colour on separate chromosome as gene for texture) yellow green Yr yR YR yr YyRr Can you think of an exception to this?

Law of Independent Assortment  Which stage of meiosis creates the law of independent assortment? Metaphase 1 EXCEPTION  If genes are on same chromosome & close together  will usually be inherited together  rarely crossover separately  “linked” Remember Mendel didn ’ t even know DNA — or genes — existed!

Sample Problem In pea plants, the green colour allele (G) is dominant over the yellow colour allele (g) for seed colour and tall (T) is the dominant plant height. A pea plant that is heterozygous for both seed colour and height is crossed with a plant that is homozygous recessive for both. Determine the genotypic ratio for their offspring.

Solution GgTtggtt GTGtgT gt GgTt x GgttggTtggtt GgTtggTtggtt GgTt GgttggTtggtt GgTtGgttggTtggtt 4/16 = 1/4 green tall 4/16 = 1/4 green short 4/16 = 1/4 yellow tall 4/16 = 1/4 yellow short 1:1:1:1 Ggtt

What is the probability of getting an offspring that are heterozygous for both characters? A: They must be GgTt 4/16 = ¼ or 25%

Any Questions? 1. Bikini Bottom 2 WS 2. Pg. 135 # (Mono) 3. Pg. 141 #1,2,4-9 (Di) Homework Quiz next class on Mono and Dihybrid Crosses!