PART IV: REVIEWING PCAT BIOLOGY
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tall : 1 short
Key: T = tall plant t = short plant
FIGURE 716: Monohybrid cross The crossing of two heterozygous individuals leads to the typical 3:1
phenotypic ration observed by Mendel
dominant phenotype, while 25% had the recessive phenotype While the recessive phenotype had disappeared in the F1 generation, it had reappeared in the F2 generation The F1 offspring were all heterozygous When two heterozygotes are bred to produce the F2 generation, the offspring will always show Mendel s observed 3:1 phenotypic ratio A cross between two heterozygotes that results in a 3:1 phenotypic ratio can be seen in Figure 716
Using a testcross (also called a backcross) is a method to determine the genotype of a parent with a dominant phenotype An organism with the dominant phenotype may be either homozygous or heterozygous In the testcross, the parent with the dominant
phenotype is always crossed to a homozygous recessive mate The outcome of the phenotypic ratio of the offspring will reveal the genotype of the unknown parent If 100% of the offspring have the dominant phenotype, then the unknown parent was homozygous dominant If the offspring display a 1:1 ratio, the genotype of the unknown parent was heterozygous
CHAP 7: GENERAL BIOLOGY
Mendel s Law of Independent Assortment
A dihybrid cross considers the inheritance of two different traits at the same time The rules of the monohybrid cross apply as long as the traits involved meet certain criteria Mendel s law of independent assortment states the following: The alleles must assort independently during gamete formation, meaning that the distribution of alleles for one trait has no in uence on the distribution of alleles for the other trait If two genes are linked that is, they occur on the same chromosome they will not assort independently and thus will be inherited together, changing the expected outcomes in the offspring
Two unlinked traits can be considered together in a Punnett square When two traits are involved in a dihybrid cross, each trait is assigned a different letter In order to predict the possible offspring, all possible gamete combinations of each trait for the parents must be considered A Punnett square is used as a matrix to match up all possible gamete combinations for the offspring Suppose two parents have the genotypes AABB and aabb All F1 offspring will be AaBb If two F1 offspring are bred, a 9:3:3:1 phenotypic ratio will be seen in the F2 generation See Figure 717 for an example of a dihybrid cross
EXCEPTIONS TO MENDEL S LAWS
While Mendel s laws tend to be good predictors of inheritance for some genetic situations, sometimes these laws do not apply Not every trait operates according to a simple dominant/recessive pattern or in a completely predictable manner
The location of a gene on a chromosome is referred to as the locus of the gene Genes that are linked occur on the same chromosome, which means that if one allele is found in a gamete, the other will be too In the case of linkage, the combination of gametes produced will not be as diverse as would be the case with nonlinked alleles In some cases, the loci of the alleles are so close together that they will always be inherited together However, if the loci of the alleles are far away from each other on the chromosome, then there is a possibility for crossing over or genetic recombination to occur
PART IV: REVIEWING PCAT BIOLOGY
LG LLGG LLGg LlGG LlGg
Phenotypic Ratio 9 long wings, gray body 3 long wings, black body 3 short wings, gray body 1 short wings, black body
Key: L = long wings l = short wings G = gray body g = black body
FIGURE 717: In a dihybrid cross, the inheritance of two unlinked traits is considered simultaneously
In this cross, Mendel s 9:3:3:1 phenotypic ratio is observed
For the traits Mendel observed with pea plants, there were always two alleles One was dominant and one was recessive However, while an individual can only inherit two alleles (one from each parent) for any given trait, there may be more than two alleles to select from in the gene pool that consists of all genotypes in the population These new alleles arise due to mutation and increase diversity in the population
Human blood type is an example of multiple alleles The ABO system has three alleles: IA, IB, and i The alleles IA and IB are dominant, while the allele i is recessive Each allele codes for either the presence or absence of particular antigens on the surface of red blood cells Any time multiple alleles are involved with a trait, more than two potential phenotypes will be expected This is the case in blood type, where four phenotypes can be observed: Type A, Type B, Type AB, and Type O