Mendelian Genetics Problems hen a genetic cross occurs beteen the gametes of to individuals, there are a number of possible combinations for the arrangement of alleles in the offspring. e can easily organize these possible combinations by using Punnett squares. Punnett Squares are used to predict both the phenotype and genotype of the offspring. The ho to: First, all of the possible gametes for one parent are listed on the top of the square and the possible gametes for the other parent are listed don the side. Each box of the square is filled by copying the ro or column head letters across or don the empty squares. For example: A cross beteen to heterozygous parents (Tt and Tt) ould look like this: NOTE: a cross beteen to heterozygous parents ill alays yield a phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1
A. Simple Dominance Problems Set up the key for the symbols Refer to p. 160 161 Use the first letter of the dominant trait A capital shos the dominant phenotype A loer case letter shos the recessive phenotype Determine the possible combinations of alleles that could be passed on Set up a Punnett square Anser the questions in the problem: pay attention to phenotype vs. genotype. hite petals coloured petals Example: In floers, hite petals are dominant to coloured. Predict the genotypes and phenotypes of the offspring if: a. a heterozygote is crossed ith a homozygous dominant b. a heterozygote is crossed ith a homozygous recessive a. b.
refer to p. 164 165 B. Dihybrid Cross hen TO pairs of characteristics are considered, it is called a dihybrid cross. In order for us to use the Punnett square to solve these problems, the La of Independent Assortment must hold true. That is, the different factors are independent of each other hen they are being sorted into egg and sperm cells. The traits separate randomly. To rite out these problems, follo the same steps as for the simple dominance problems. Decide on hich trait you ill rite first and be consistent! If a capital letter is being used for that trait, it should be ritten first. R r round rinkled Y y yello green Example: Round seeds are dominant to rinkled. Yello seeds are dominant to green. To plants are crossed: one is homozygous for both round and yello traits, the other has a rinkled, green appearance. a. First sho the genotypes of the Fl generation. R R Y Y r r y y b. Then sho the genotypes and phenotypes in the F2 generation. r y r y r y r y R Y R Y R Y R Y R r Yy
refer to p. 166 167 C. Test Crosses If the genotype of an individual is unknon, a test cross may be performed to determine hat the genotype is. A test cross involves crossing the individual ith the unknon genotype (that shos a dominant trait) ith an individual that is homozygous recessive for that trait. The phenotype expressed in the offspring may then be used to ork backards to determine hether the unknon genotype is homozygous dominant or heterozygous. Example: hite petals are dominant to red. A hite plant is test crossed. All the Fl ere hite. hat is the genotype of the hite plant? Test against homozygous recessive: hite red??
D. Incomplete Dominance In some species the dominant traits do not completely hide the expression of the recessive traits. A heterozygous individual has a different phenotype, somehere in beteen. B black hite Example: In Andalusian fols, the genotype BB gives black feathers. B gives blue tinted feathers, hile gives hite feathers. To blue feathered fols ere crossed. hat are the phenotypic and genotypic ratios of the F1 generation? B B BB B B
E. Codominance Codominance allos both alleles to be expressed at the same time. R red hite Example: The colour roan in horses and bulls is a combination of red and hite hairs. In this case the hair is either red or hite in colour and both colours are present. There are no hairs that appear to be a combination of both colours in one hair. The genotype RR gives red hair. R gives roan hair and gives hite hair. A red haired horse and a hite haired horse ere crossed. hat are the genotypic and phenotypic ratios of the Fl generation? R R R R R R
F. Sex linked Traits Some traits are sex linked; the alleles travel on the chromosome only. Therefore, a female ill have 2 alleles one on each chromosome. A male has only one chromosome, so that allele given by the mother determines hat he ill have for that trait. As a result, the recessive phenotype is seen more often in males. Examples of human sex linked traits: Duchenne Muscular Dystrophy Colour Blindness Hemophilia The key for this type of problem must clearly sho the ith the trait, and the Y ith no trait. Make sure that you set it up properly! Example: A mother is a carrier for colour blindness. The father has normal vision. hat ould be the phenotypic ratios of their children? Y Y b b b Y b Y normal colour blind normal (male)
G. Multiple Alleles In these types of problems, there are more than just to possibilities involved. For example, there are three alleles involved in blood types. Possible blood types: A, B, AB, or O. Phenotype (Blood Type) A B AB O Genotype I A I A or I A i I B I B or I B i I A I B ii I AA I B i "A" marker "B" marker no marker Example: Fred has blood type AB. He marries ilma, ho has type A blood. ilma s mother as type 0. hat is the chance that their first child ill have type B blood? Could you predict the name of their first child? I A I A I A I A i I A i I B I A I B I B i