Mendelian Exceptions

- Not all genes show simple patterns of dominant and recessive alleles

•Because the majority of traits are controlled by more than one gene/alleles

Objective: F3 - Predict possible outcomes of non-Mendelian inheritance & explain how sex is determined

Codominance

•Cases in which both alleles are expressed•White cow x Brown cow =

Speckled (white and brown) cow

Cow-1.au

Incomplete dominance Neither allele is dominant •Red flower x White flower = Pink flower

• In a certain cactus, prickly spines can be two pronged or one pronged. If a true breeding (homozygous) one-pronged cactus is crossed with a true breeding two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged.

• Is this an example of codominance or incomplete dominance?

codominance

Practice Problems

• In snapdragons, the combined expression of both alleles for flower color produces a new phenotype that is pink. This illustrates incomplete dominance. The Punnett square above shows that both the white and red snapdragons are homozygous. Which of the following would be the correct product from a cross between two heterozygous pink snapdragons? [VA05 EOC]

• A 1 red, 2 pink, 1 white• B 2 red, 2 white• C 2 red, 1 pink, 1 white• D 1 red, 1 pink, 2 white

Sex-linked Genetics

Sex DeterminationSex Determination• If you are female,

your 23rd pair of chromosomes are homologous, XX.

• If you are male, your 23rd pair of chromosomes XY, look different.

X XFemale

YXMale

XX Female

XY Male

X

X

X Y

XX Female

XY Male

XX Female

XY Male

•Traits controlled by genes located on sex chromosomes are called sex-linked traits.

•The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes.

Sex-linked inheritanceSex-linked inheritance

Sex Linked Traits• When genes are located on the X

chromosome, females receive two alleles for these genes, but males only receive one.

• In males, the genotype is automatically known.• Colorblind male - XbY (recessive)• Normal male - XBY (dominate).

• A female can be:• XBXB - normal• XBXb - carrier• XbXb - colorblind

• The following shows a cross between a normal man and a woman who is a carrier.

50%

• Based only on the sex chromosomes in typical human egg and sperm cells at fertilization, the probability of producing a female is -

[CA EOC]

0%

• In certain species of roses, white roses and red roses are incompletely dominant to each other. When a red rose and a white rose are crossed, a pink rose is produced. What is the probability of producing a white rose when a red rose is crossed with a pink rose?

25%

• The allele for having a bent pinky finger is dominant to the allele for having a straight pinky finger. If two people who are both heterozygous for the trait of a bent pinky finger have a child, what is the percent probability that the child will have straight pinky fingers?

• Polycystic kidney disease is a dominant trait. What are the chances that a child will develop polycystic kidney disease if one parent is heterozygous and the other is normal?

50%

• A gene that is sex-linked is BEST described as which of the following? •A It results in all male offspring. •B It results in all female offspring. •C It is located on the X

chromosome. •D It is located inside the

mitochondria.

[GA04 EOC]