Genetic Experiments

Genotype and Phenotype

            a) Give the genotype of an individual, who is homozygous dominant for Brown eye color,

            where “B” is the letter used to distinguish this trait.

            b) Give the genotype of an individual who is homozygous recessive for brown eye color.

            In the absence of melanin brown pigment, the iris is blue.

            c) Give the genotype of an individual who is heterozygous for the brown eye color trait.

            d) Assuming complete dominance-recessive, give the phenotypes for the following

                        Bb :________________________

                        bb :________________________

                        BB:________________________

Once an individual’s genotype is known, they can be theoretically crossed to create possible offspring.  Genotypes, phenotypes, and their ratios can then be established.   The first generation is the parent (P1) and second generation of offspring are labeled as filial or F1.  Other generations of offspring are labeled accordingly, such as F2 and F3.  Remember, inheritance patterns will affect how a trait is expressed in the phenotype.

Monohybrid cross

            If a women and her husband are both heterozygous for Brown eye color.  This trait is considered to be a complete Dominance-Recessive inheritance pattern.   What is the genotype and phenotype of their first generation offspring, F1?   Use a punnet square to show your work.

                                                            Mother’s gametes

Genotypical ratio :        

Phenotypical ratio :

Monohybrid cross

A rare allele for extra digits (polydactyla) in the human behaves as a dominant (E) to the normal number of digits (e).  Complete a punnet square for the following problem.  Give the genotypes of the parents and offspring for F1 and F2 as follows.

A man with an extra toe on each foot marries an unrelated women with the normal condition.

The first child has an extra finger; the second child is phenotypically normal.  In time, the “normal” offspring marries a normal, unrelated women and produces a child with an almost complete extra toe on one foot and the normal number of toes on the other.

Parent (P1) genotypes mother ________________                father________________

Offspring (F1) genotypes        1^st child ________________              2^nd child ______________

Parent (P2) genotype               mother _________________              father _________________

Offspring (F2) genotype                                  Child ___________________

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Examples of Dominant - Recessive Traits:

Record your phenotypical traits. Compare to family memers, if possible.

My traits :__________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

Dihybrid Crosses

Dihybrid crosses can done and will involve two traits.  An expanded punnet square can be used to help with the possible F1 outcomes, once the parent’s genetic traits are known.  You will be finding the genotypes and phenotypes for the following study.  Assume these traits are incomplete dominant-recessive, meaning the recessive gene can be expressed phenotypically.

In humans, the dominant D and E are BOTH required for normal development of the cochlea and auditory nerves, respectfully.  The recessives, d and e, can result in deafness due to an impairment  of the development of these essential parts of the inner ear.  Complete a punnet square for the following crosses showing the genotype and explain the phenotypes of the F1 generation.

            A) DDee x ddEE

            B) DdEE x DDEe

            C) DdEE x DdEe

            D) DdEe x DDEe

Remember to determine the genetic makeup of the mother and father’s possible gametes first in order to insure proper set up of the Punnet square.  For example, in the first exercise, the mother’s gametes can only be De and the fathers can only be dE.  Phenotypically, the mother has normal cochlea (DD), with impaired nerve function(ee), and the father has an defective cochlea (dd), but normal auditory nerves (EE).  Because both mechanisms for hearing are not in tack, both parents are deaf.

As you have seen, dihybrid crosses involve multiple alleles.  Another example is seen in the human blood types, A, B, AB, and O.  Multiple alleles can also be expressed in the co-dominance inheritance pattern.  For blood type A, the genotype is either AA or Ai, where i is the recessive.

For the blood type B, the genotype is either BB or Bi.  The blood type AB has only one genotype, AB, since both are dominant and expressed.  The blood type O is recessive and its genotype is ii.

Do the dihybrid parent, P1, crossed to find the offspring, F1, possible genotypes for the following :

            A man with AB blood marries a women with AB blood.

            A man with type O blood marries a women heterozygous for type A blood.

            A man with blood type B marries a women with blood type A.

            A man homozygous for type A blood marries a women homozygous for type B blood.

Before the use of DNA in paternity tests, blood tests were used to help rule in or rule out the man as the father.  A women with type B blood has a child with A blood.  What possible blood type(s) could the father be ?

a) AB              b) O (ii)           c) AA              d) BB              e) Ai                f) Bi

Sex Linked Traits

            Sex linked traits involve the sex chromosomes, XX for the female and XY for the male.

The X or Y is used in the determination of the phenotype, as well as the genotype.  Traits are identified as a superscript next to the sex chromosome, and will have either a dominant capital letter or  recessive lower case letter to help identify the phenotypical expression.  Sex linked is usually on the X chromosome, unless otherwise specified and usually a complete dominance inheritance pattern.

In humans, a certain rare sex-linked recessive “i” can result in a cleft iris of the eye.  Its allele “I” is required for normal iris development.  Determine the genotype of the parents and the genotype and phenotype of their offspring for the following crosses:

                        A) A man with a cleft iris and a woman who carries only the normal allele.

                        B) A woman with a cleft iris and a man with a normal iris.

                        C) A woman who is heterozygous for the trait and a man with a cleft iris.

                        D) A woman who is heterozygous for the trait and a man with a normal iris.

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In humans, a sex-linked dominant allele R produces a type of rickets (skeletal defect).  

The recessive allele, r, is associated with normal skeletal development. 

Give the results expected from the following crosses :

                        A) Man with rickets and a woman who does not have this condition

                        B) A woman who has rickets, but whose father did not, and a man who does

                                    not have the condition.

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In humans, green color blindness is X linked.  The dominant allele, D, represents a normal color distinction and the recessive, d, codes for deutan color blindness. 

What are the possible outcomes of the following crosses :

                        A) color blind man and a women with normal vision

                        B) normal visioned male with a women who is a carrier for the trait

                        C) color blind man and a women who is a carrier for the trait

                        D) From your results, can a women ever be color blind ?

Genetics problems can become even more complicated when autosomal and X linked traits are followed.

In humans, the autosomally linked allele “A” is required for normal skin pigmentation.  Its recessive allele “a” is associated with the albino or lack of pigment condition.  Several pairs of alleles affect color vision.  One of the X-linked recessives responsible for red-green color blindness may be represented by “p”.  Its dominant allele P is required for normal color vision.

Consider pattern baldness in the human to result from the expression of the autosomal factor “B” as a dominant in males and a recessive in females.  Its allele “b” (nonbald) behaves as a dominant only in females.

Do the following cross considering all alleles mentioned and give the F1 genotypes and phenotypes for the following:

 Nonbald, color blind, normal pigmented male marries a nonbald, normal pigmented woman with normal vision, whose father was an albino color-blind man and whose mother had normal skin pigmentation, but went bald later in life.

                                    Husband’s Genotype   ___________________________

                                    Daughter’s Genotype   ___________________________

                                    Father’s Genotype       ___________________________

                                    Mother’s Genotype      ___________________________

                                    F1 cross