HEREDITY: Going beyond Mendel Individuals don’t always follow the basic pattern of inheritance (dominant/recessive)
Many inheritance patterns are more complex: • Linked genes • Incomplete/codominance • Multiple alleles • Sex Determination/Sex-linked traits • Polygenic Inheritance • Effect of environment on gene expression
Independent Assortment • Recall……..Genes on separate chromosomes will be inherited separately. • The way in which chromosomes line up during meiosis is random • This adds to variation
LINKED GENES • Chromosomes carry MANY genes, so some genes will be on the same chromosome. • Genes on the same chromosome are called LINKED genes and are usually inherited together. • I say usually because what can happen during Prophase I of meiosis?
Incomplete dominance • Neither allele completely masks the expression of the other • Heterozygous individuals have a different phenotype than either parent • There is an intermediate phenotype, sometimes called blended inheritance
Example of incomplete dominance: • The snapdragon • The red snapdragon is homozygous and the white snapdragon is also homozygous. The red does not completely mask the white, so a pink phenotype is the result of a cross between red and white. For this cross, we can use different letters: • RR x WW = RW
In humans, spacing of eyes and wavy hair are examples of incomplete dominance
Co-dominance • Similar to the concept of incomplete dominance. No allele masks the other • In co-dominance, BOTH traits show in the phenotype • Example-barred rock chicken is a result of a cross between a black chicken and a white chicken
Example of co-dominance: • In humans, sickle cell anemia • as well as types A and B blood show co-dominance
Multiple Alleles • In some cases, an offspring inherits two alleles from the parent from a choice of three or more alleles • Example is pigeon feather color Blue is recessive to red, dominant to choclate Chocolate is recessive to both Red can AA, Ab or Ac Blue can be bb or bc Chocolate will be cc Red is dominant to other alleles
Examples of multiple alleles: • In humans, A, B, o blood types (not the positive/negative factors)
Cross a father heterozygous for B blood with a mother heterozygous for A blood: • IB i x IA i
Polygenic Inheritance • Inheritance pattern controlled by many genes (do not confuse this with multiple alleles) • A trait controlled by polygenic inheritance could have MANY phenotypes • Results will show a bell curve or a continuum
Example of polygenic inheritance: • Height and skin color in humans
Sex-linked Inheritance • Sex-linked traits are controlled by genes found on a sex chromosome • The traits we study are found on the X chromosome. The X chromosome carries different genes than the Y chromosome carries.
Sex-linked traits in humans • Include red-green color-blindness and hemophilia • These disorders are found more often in males because they just need one copy of the defective allele to have the disorder. • Males do not have another copy of the genes on the X chromosome because they have only one X chromosome.
GENE EXPRESSION • Gene Expression relates to the presence or dimension of the appearance of a trait for which the genes are present. • Internal environment (hormones, nutrition) and external environment (temperature) may have an effect on whether a gene is expressed and if so, how much it is expressed.
Examples of gene expression: • Horn size in Bighorn sheep Genotypes are the same, but phenotypes are different, due to ?
Gene expression can also be- • Affected by temperature, as in a weasel He is called an ermine in the winter He is called a stoat in the summer