11-3 Exploring Mendelian Genetics More Mendelian Genetics

Single-Factor Crosses Previous cross-breeding examples we’ve seen have involved just one factor/trait. TRAIT: heightALLELES: giant (G) / dwarf (g) Gg G g gg g Note that the P generation plants are HR and HD. Organisms that are homozygous for a trait are also known as true-breeding.

 Mendel also crossed true-breeding plants that produced round, yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled, green peas (genotype rryy).  This is important because there are two traits being crossed instead of just one.  All of the F 1 offspring produced round, yellow peas (RrYy). Two-Factor Crosses Seed shape Seed color Round Wrinkled Yellow Green

Independent Assortment  To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross.  Because each allele is slotted into its own row or column in the Punnett square, a two-factor cross requires a 4x4 square. Two-Factor Crosses

The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). Two-Factor Crosses

If the pea plant is HD for seed shape, its alleles are RR. If the pea plant is HR for seed color, its alleles are YY. The plant has a total of 4 alleles when considering two traits: RRYY RRYY RY RY

Two-Factor Crosses Mendel’s crossing of the two true-breeding pea plants in the P generation yielded F 1 generation plants that were all heterozygous.

 Mendel crossed the heterozygous F 1 plants (RrYy) with each other to determine how the alleles would segregate from each other in the F 2 generation. Two-Factor Crosses The Punnett square predicts a 9 : 3 : 3 : 1 ratio in the F 2 generation.

 In Mendel’s experiment, the F 2 generation produced the following:  some seeds that were round and yellow  some seeds that were wrinkled and green  some seeds that were round and green  some seeds that were wrinkled and yellow Two-Factor Crosses

What inheritance patterns exist aside from simple dominance? Incomplete Dominance – one allele is not completely dominant over another Codominance – both alleles contribute equally to the phenotype Multiple Alleles – more than two alleles exist for a trait, i.e. red/green/yellow toothpick fish or brown/green/blue/etc. eye color Polygenic traits – traits controlled by two or more genes Special Cases of Alleles

When one allele is not completely dominant over another, it is called incomplete dominance. In these cases, the hybrid phenotype is a combination of the homozygous phenotypes. Incomplete Dominance RED + BLUE  VIOLET

WW RR Incomplete Dominance A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). Both alleles are dominant, so both are indicated using capital letters.

Codominance In codominance, like in incomplete dominance, both alleles contribute to the phenotype. The difference is that the resulting phenotype is rarely a combination of traits, but instead BOTH TRAITS EXPRESSED SEPARATELY. The horse’s brown and white colors do not blend to form a new color, but appear separately.

 Genes that are controlled by more than two alleles are said to have multiple alleles.  An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.  What are some traits in human beings that involve multiple alleles? Eye Color Hair Color Blood Type Multiple Alleles

Different combinations of alleles result in different colors. Full color: CC, Cc ch, Cc h, or Cc Chinchilla: c ch c h, c ch c ch, or c ch c Himalayan: c h c, or c h c h AIbino: cc KEY C = full color; dominant to all other alleles c ch = chinchilla; partial defect in pigmentation; dominant to c h and c alleles c h = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Multiple Alleles

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 Polygenic Traits  Traits controlled by two or more genes are said to be polygenic traits.  Skin color in humans is a polygenic trait controlled by more than four different genes. Polygenic Traits

11–3 Copyright Pearson Prentice Hall

11–3  In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F 2 generation is evidence for  the two traits being inherited together.  an outcome that depends on the sex of the parent plants.  the two traits being inherited independently of each other.  multiple genes being responsible for each trait.

11–3  Traits controlled by two or more genes are called  multiple-allele traits.  polygenic traits.  codominant traits.  hybrid traits.

11–3  In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have  pink flowers.  white flowers.  half white flowers and half red flowers.  red flowers.

11–3  A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of  incomplete dominance.  multiple alleles.  codominance.  a polygenic trait.

11–3  Mendel's principles apply to  pea plants only.  fruit flies only.  all organisms.  only plants and animals.