Dihybrid Crosses and Law of Independent Assortment

Dihybrid Crosses andLaw 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 SegregationReview • 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 andseed shape • dihybrid crosses Mendelwas working outmany of the genetic rules!

F1 generation (hybrids) yellow, round peas 100% F2 generation Dihybrid cross P true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr 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

YR YR yR Yr yr yr 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? Is it this? Or this? YyRr Y and R alleles transmitted independently Which systemexplains the data? Y and R alleles transmitted together

9/16 yellow round YyRr YyRr 3/16 green round YR yr YR yR Yr yr 3/16 yellow wrinkled 1/16 green wrinkled Is this the way it works? or YyRr x YyRr What was actually observed YR yr Well, that’sNOT right! YR YYRR YyRr yr YyRr yyrr

9/16 yellow round YyRr YyRr 3/16 green round YR YR yr YR yR Yr yr Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross or YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr BINGO! YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

yellow green round wrinkled Can you thinkof an exceptionto this? Mendel’s 2nd law of heredity • 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) YyRr Yr Yr yR yR YR YR yr yr

Metaphase 1 Law of Independent Assortment • Which stage of meiosis creates the law of independent assortment? RememberMendel didn’teven know DNA—or genes—existed! EXCEPTION • If genes are on same chromosome & close together • will usually be inherited together • rarely crossover separately • “linked”

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.

gt gt gt gt Solution 1:1:1:1 GgTt ggtt x 4/16 = 1/4 green tall GT Gt gT gt 4/16 = 1/4 green short GgTt Ggtt ggTt ggtt Ggtt GgTt ggTt ggtt 4/16 = 1/4 yellow tall GgTt Ggtt ggTt ggtt 4/16 = 1/4 yellow short GgTt Ggtt ggTt 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 WS2. Pg. 135 # 10-14 (Mono)3. Pg. 141 #1,2,4-9 (Di)Homework Quiz next class on Mono and Dihybrid Crosses!

Dihybrid Crosses and Law of Independent Assortment

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