Test Crosses & Dihybrid Crosses

Independent assortment and Segregation.. Y=yellow, y=green, R=round, r=wrinkled According to the Law of Independent Assortment, a plant that is hybrid for 2 traits will form FOUR different gametes Y y R r YR Yr yR yr Independent assortment and Segregation..

Possible gametes Dihybrid Cross

EXAMPLE: #1 What are the possible gametes for the following parents (B=black coat, b= white, R= regular eyes and r= bulging eyes); a) BBRr b) BbRr c) bbRR

Therefore, Probabilities…

NOTICE: HETEROZYGOUS CROSS HAS RATIO OF 9:3:3:1

Was the inheritance of one character affected by the inheritance of another? It appears that the inheritance of seed shape has no influence over the inheritance of seed colour. The two characters are inherited INDEPENDENTLY. The pairs of alleles that control these two characters assort themselves independently = Mendel's Second Law, THE LAW OF INDEPENDENT ASSORTMENT.

EXAMPLE #2: A pea plant that heterozygous for the traits tall (T) and round seeds (R), is crossed with a dwarf with wrinkled seeds. Determine the phenotype and genotype ratios of the F1 generation

P: _________________________ P phenotype: ______________________ Gametes: __________________________ F1:

EXAMPLE #3: Yellow seeds (Y) are dominant over green seeds (y). Round seeds (R) are dominant over wrinkled seeds (r). If two plants that are heterozygous for BOTH traits (dihybrid) are crossed, what will be the phenotype and genotype ratios of the F1 generation?

P: _________________________ P phenotypes: _______________________________ Gametes: ____________________________________ F1

ONE PARENT IS ALWAYS HOMOZYGOUS RECESSIVE!!!!!! A cross between an individual exhibiting the dominant phenotype of a trait and an individual that is homozygous recessive for that trait in order to determine the genotype of the dominant individual. Recall: A dominant phenotype can be either homozygous dominant (HH) or heterozygous (Hh) ONE PARENT IS ALWAYS HOMOZYGOUS RECESSIVE!!!!!! Test crosses

When a dominant phenotype is crossed with a homozygous recessive any hidden traits can be revealed We can determine the genotype of the unknown parent by looking at the ratios expressed in the offspring

Possible outcomes…Mono If the parent is PP then the phenotype of all the offspring; Will be ALL purple Possible outcomes…Mono

Possible outcomes…Mono If the parent is Pp then the phenotype of all the offspring; Will be purple and white Possible outcomes…Mono

Example Character: Coat colour in mice Traits Alleles Genotypes Phenotypes Grey G GG Grey White g Gg Grey gg White Grey mice could have one of two different genotypes, GG or Gg. If they are crossed with a white mouse (gg) these genotypes will give two different results.

In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross. This is done with a homozygous recessive (rr) individual. The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy). Dihybrid test cross…

Possible outcomes Parent YYRR all offspring are yellow and round Parent YyRR  offspring will be yellow or green AND all round Parent YYRr  offspring will be all yellow AND round or wrinkled Parent YyRr  offspring will be yellow or green AND round or wrinkled

Offspring all yellow/round PARENT YYRR Offspring all yellow/round

Offspring: yellow/round or green/round PARENT YyRR YR yR yr YyRr yyRr Offspring: yellow/round or green/round

Offspring: yellow/round or yellow wrinkled PARENT YYRr YR Yr yr YyRr Yyrr Offspring: yellow/round or yellow wrinkled

OFFSPRING: Yellow/round, yellow/wrinkled, green round, green wrinkled PARENT YyRr x yyrr OFFSPRING: Yellow/round, yellow/wrinkled, green round, green wrinkled

When doing an example test cross; State your variable designation Write down your predicted genotype Do a Punnet square to prove your prediction Write a statement

Ex. In the “chocolate moose” brown fur is dominant to yellow fur and round ears are dominant to pointed ears. If a brown and round eared moose is test crossed and the offspring are as follows, what were the genotypes of the original parent? 5 brown round 2 yellow round 2 brown pointed 1yellow pointed

In a test cross to determine whether a fruit fly is homozygous (WW) or heterozygous (Ww) for long wings, 7 long-winged flies & 1 short winged-fly are produced. Which is a valid conclusion? A. the unknown fruit fly is WW B. the unknown fruit fly is Ww C. the unknown fruit fly is ww D. more offspring are needed

In hamsters, long tails (L) are dominant to short tails (l) In hamsters, long tails (L) are dominant to short tails (l). A student wishes to perform a test cross to determine whether a female long-tailed hamster is homozygous or heterozygous for long tail length. She mates the hamster with a male long-tailed hamster & studies the offspring, which are 100% long-tailed. She conclude that the female hamster's genotype is "LL". What mistake(s) did the student make? A. she should have mated the female hamster with a male that was known to be hybrid B. she should have mated the female hamster with a short-tailed male hamster C. she should have mated the female hamster with another long-tailed female D. she has to mate members of the litter before she can make a conclusion