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Beyond Mendel. Incomplete dominance, Multiple alleles, Co-dominance, Polygenetic traits Created by Susan Smith Heritage High School. Incomplete Dominance. Definition: A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype.
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Beyond Mendel Incomplete dominance, Multiple alleles, Co-dominance, Polygenetic traits Created by Susan Smith Heritage High School
Incomplete Dominance • Definition: A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype.
Incomplete Dominance • Some alleles are not dominate over other alleles • Heterozygous genotype do not mask the recessive allele • Heterozygote alleles will blend to form an intermediate phenotype • Example is snapdragons flowers
Incomplete Dominance • If you cross a Red snapdragon (RR) with a White snapdragon (WW) you will get a pink flower (RW)
Incomplete Dominance • Sample Questions1. A cross between a blue blahblah bird & a white blahblah bird produces offspring that are silver. The color of blahblah birds is determined by just two alleles. a) What are the genotypes of the parent blahblah birds in the original cross? b) What is/are the genotype(s) of the silver offspring? c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?
Co-Dominance • Definition: Co-dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
Co-Dominance • The prefex “Co” means together • A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. • So in co-dominance the heterozygote shows a different phenotypes that consists of the dominant and the recessive alleles together on the same individual • They do not blend but each are expressed
Co-Dominance • R = red flowers • W = white flowers • red x white ---> red & white spotted • RR x WW ---> 100%
Co-Dominance • A very very very very very common phenotype used in questions about co-dominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of co-dominance.
Co-Dominance Sample Questions Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur? A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?
Multiple alleles Definition: Any of a set of three or more alleles, or alternative states of a gene, only two of which can be present in a diploid organism. Even though there are more than two possible alleles for the trait; each individual may only have two of the alleles at a time, one from each parent
Multiple Alleles • Mendel's concept of inheritance dealt with only two different alleles for a particular gene. • After the rediscovery of Mendel's work, it didn't take long before genes were discovered that had more than two possible alleles. • Some genes have three, four, or more alleles. • An extreme example of this is the white gene, one of the genes controlling eye color in Drosophila. So far, over 100 different alleles of this gene have been identified! Of course, any individual fruit fly would only have a maximum of two of those alleles.
Multiple Alleles • Understandably, having more than two alleles makes genetic investigation more complicated. We can still use Mendel's basic rules to understand inheritance of the trait specified by a multi-allelic gene. Let's use coat color in rabbits as an example. The gene for coat color has four alleles: • wild type (c+): chinchilla (cch): himalayan (ch): albino (c): • Which of the alleles is dominant, and which is recessive?
Multiple Alleles • The albino allele is recessive. • The Himalayan allele is dominant to albino, but recessive to everything else. • Chinchilla is partially dominant to himalayan and albino, but recessive to wild type. • Wild type is dominant to all of the other alleles. This can be shown in the following way: • c+ > cch > ch > c
Multiple Alleles • The phenotypes can be determined from the genotype using Mendelian principles, because any individual will only have two of the alleles. • For example, a rabbit with the genotype c+ ch would have a wild-type phenotype, because the wild-type allele is dominant to Himalayan. • A rabbit with the phenotype ch c would be himalayan, because the Himalayan allele is dominant to albino. • A rabbit with the genotype cch ch would be chinchilla with Himalayan markings, because chinchilla is only partially dominant to Himalayan. • What phenotype would a c+ cch rabbit have?
Co-Dominance and Multiple Alleles • It is also possible to have multiple alleles of a gene that exhibits co-dominance. • The best-known example of this is the human ABO blood group. Everyone knows that there are four blood types within this group: A, B, AB, and O • The different blood types are defined by the presence of different antigens on the surface of the erythrocytes (red blood cells). The gene responsible for producing these cell-surface antigens is called I. • This gene has three alleles: IA, IB, and IO. • IA and IB are co-dominant, and both are dominant to IO. • Therefore, a person with type A blood can be IA IA or IA IO, a person with type B blood can be IB IB or IB IO, a person with type AB blood is IA IB, and a person with type O blood is IO IO.
Polyogenetic traits • Traits that are controlled by more than one gene pair • Several pairs of genes may be involved in determining the phenotype of the individual • Such traits produce continuous variation Which can be represented in bell shaped curve • Example: human height
Linked Genes • Chromosomes assort independently, not individual genes • When genes are far apart on a chromosome then they can be moved by crossing over • When genes are close together on a chromosome then they will stay together
X- linked Genes • X chromosome is much larger than the Y chromosome (size does matter) • Female get two x chromosomes two copies of each gene • Males get only one copy of the X chromosome and one y chromosome • For male there is not another allele on the y to match all of the alleles on the X chromosome • Males can have only one recessive allele and express the trait
X-linked Traits • X linked Traits • Color Blindness
