S OLVING H YBRID C ROSS P ROBLEMS 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. 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 = 1 st Parents) W w w F1 = 1 st Offspring WWWw ww Phenotypes Widows peak =75% Straight line = 25% Genotypes WW =25% ww=25% Ww=50% W 1 st Law – ½ chance for each 2 nd Law – ½ X ½ = ¼ 3 rd Law ½ X ½ + ½ X ½ = ½

P UNNETT S QUARE P RACTICE  Use a Punnett Square to determine the phenotype and genotype ratios of a cross between: 1. Bb X bb 2. TT X tt 3. Tt X Tt

R ESULTS OF P UNNETT S QUARES TtTt TtTt BbBbbbbb b b T T B t b t 1. 2.

P UNNETT S QUARE R ESULTS 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 = tallG = green t = shortg = 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 = tall Black= short

B EYOND D OMINANT AND R ECESSIVE A LLELES 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

B EYOND D OMINANT AND R ECESSIVE A LLELES 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

C ROSS 2 R OAN C ATTLE R W R W RR (red) Rr (Roan) Rr (White)

B EYOND D OMINANT AND R ECESSIVE A LLELES 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 B LOOD G ROUP 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.

G ENOTYPES & P HENOTYPES GenotypePhenotype AA or AOType A BB or BOType B ABType AB OOType O

P RACTICE P UNNETT S QUARES FOR B LOOD T YPE I NHERITANCE 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 AB 2. ½ AB and ½ A 3. ¼ AB, ¼ A, ¼ B and ¼ O

B EYOND D OMINANT AND R ECESSIVE A LLELES 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 groups This 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.