s olving h ybrid c ross p roblems punnett squares – used to predict the percentages of genotypes...
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SOLVING HYBRID CROSS PROBLEMS Punnett Squares – used to predict the
percentages of genotypes and phenotypes of crosses
Ex. Widows Peak X Straight line Widows Peak is dominant = W
Straight line is recessive = w
Phenotype v. GenotypePhenotype v. Genotype
Phenotype = physical appearance example: widows peak or straight line
Genotype = genetic makeup example: Ww, WW, or ww
Genotypes can be homozygous (purebred)or heterozygous (hybrid)
Homozygous Widows Peak = WW Homozygous Straight line = ww Heterozygous Widows Peak = Ww
What will the genotypes and phenotypes be of a cross of
heterozygous parents?
Ww X Ww (P1 = 1st Parents)
W
w
w F1 = 1st Offspring
WW Ww
Ww ww
Phenotypes
Widows peak =75%
Straight line = 25%
Genotypes
WW =25%ww=25%Ww=50%
W1st Law – ½ chance for each
2nd Law – ½ X ½ = ¼
3rd Law½ X ½ + ½ X ½ = ½
PUNNETT SQUARE PRACTICE
Use a Punnett Square to determine the phenotype and genotype ratios of a cross between:
1. Bb X bb 2. TT X tt3. Tt X Tt
RESULTS OF PUNNETT SQUARES
TtTt TtTt
TtTt TtTt
BbBb BbBb
bbbb bbbb
b b
T T
B
t
b
t
1.
2.
PUNNETT SQUARE RESULTS
TTTT TtTt
TtTt tttt
T
3.tT
t
DIHYBRID CROSSES – TESTS HOW TWO DIFFERENT TRAITS ARE INHERITED
Led to Principle of Independent Assortment - genes for different traits
usually segregate into gametes independently of one another.
TWO-FACTOR CROSS
Cross two plants that are heterozygous for both pod color (G) and height (T)
TtGg X TtGg
Meiosis produces the following gametes:TG, Tg, tG, tg
T = tall G = greent = short g = yellow
TTGG TtGG TTGg TtGg
TtGG
ttGG TtGg ttGg
TTGg TtGg TTgg Ttgg
TtGg ttGg Ttgg ttgg
TG tG Tg tg
TG
tG
Tg
tg
Red = tallBlack= short
BEYOND DOMINANT AND RECESSIVE ALLELES Incomplete Dominance – Cases in
which one allele is not completely dominant over another. The heterozygous phenotype is somewhere in between the two homozygous phenotypes.
Ex. Four o’clock plants: Homozygous are white or red Heterozygous are pink
BEYOND DOMINANT AND RECESSIVE ALLELES Codominance – Both alleles contribute to
the phenotype of the organism.Ex. Hair color in cattle:
Homozygous are red or white Heterozygous are roan (pinkish
brown) – a mixture of red and white hairs
CROSS 2 ROAN CATTLE
R W
R
W
RR (red)Rr (Roan)
Rr (Roan) Rr (White)
BEYOND DOMINANT AND RECESSIVE ALLELES Multiple Alleles – Many genes have more
than two alleles. An individual organism has only two alleles, but more than two possible alleles exist in the population.Ex. 1. Coat color in rabbits:
Four alleles exist for one gene. This results in four possible coat
colors.Ex. 2. Genes for blood type in humans
ABO BLOOD GROUP
Blood type is an example of a trait with multiple alleles: A, B, O
A and B are co-dominant, O is recessive Type O is the Universal Donor – all blood
types can receive type O blood. Type AB is the Universal Recipient – can
receive blood from all other blood types.
GENOTYPES & PHENOTYPES
Genotype Phenotype
AA or AO Type A
BB or BO Type B
AB Type AB
OO Type O
PRACTICE PUNNETT SQUARES FOR BLOOD TYPE INHERITANCE
1. What is the probable genotype ratio among children born to an AO mother and an AB father?
2. One parent has type A blood and the other has type B. What are their genotypes if they produced children who were:
1. All AB2. ½ AB and ½ A3. ¼ AB, ¼ A, ¼ B and ¼ O
BEYOND DOMINANT AND RECESSIVE ALLELES Polygenic Traits – Traits controlled by two or
more genes.Ex. 1. Three genes control eye color in fruit
flies. Different combinations of alleles for these genes produce different eye colors.
Ex. 2. Skin color in humans is probably controlled by more than four genes.
Ex. 3. Eye color in humans
GENES AND THE ENVIRONMENT
Environmental conditions can affect gene expression and influence genetically determined traits Ex. Western White Butterfly – those that hatch
earlier have more pigment in wings – increases body temperature so they can fly
GENE LINKAGE
Alleles of different genes on the same chromosome tend to be inherited together – linkage groupsThis was missed by Mendel – he happened to
study traits located on different chromosomes.Led to clarification that chromosomes assort
independently Alleles of genes far apart on chromosomes can
assort independently because of crossovers. Frequency of crossovers can be used to determine
location of genes on chromosome
The more frequent recombination is between two genes, the more likely those genes are far apart on the chromosome.