Lab 8: Human Genetics
- To determine genotypes by observing phenotypic traits in a family
- To create pedigrees of a phenotypic traits
- To determine autosomal dominant, autosomal recessive, and sex-linked inheritance patterns by examining pedigrees
- family members (parents, siblings, children, aunts, uncles, cousins, etc.)
Description of the problem
The genotype is the specific genetic makeup of an individual, whereas the phenotype describes the physical appearance of an individual. If an individual is homozygous dominant (AA) or heterozygous (Aa), the dominant phenotype will be displayed. However, if an individual is homozygous recessive (aa), the recessive phenotype will be displayed.
Experiment: Determining genotypes of autosomal traits
The following are all autosomal traits (i.e. they’re carried on the autosomes, which include all chromosomes other than the sex chromosomes). Determine your genotype for each of the following traits. If you display the dominant phenotype (A-), determine what the “-” allele must be by examining the same trait in other members of your family. In some cases you’ll be able to determine what the “-” is, but in others you may not be able to.
1. Tongue rolling: ability to roll tongue = dominant, no ability to roll tongue = recessive
2. Earlobe attachment: unattached earlobes = dominant, attached earlobes = recessive
3. Skin pigmentation: freckles present = dominant, freckles absent = recessive
4. Hairline: widow’s peak present = dominant, widow’s peak absent = recessive
5. Little finger alignment: end of little finger bends toward ring finger = dominant, little finger is straight = recessive
6. Length of big toe: big toe shorter than the second toe = dominant, big toe longer than second toe = recessive
7. Dimples: dimples present = dominant, dimples absent = recessive trait
8. Interlocking fingers: Fold your hands together by interlocking your fingers. Which thumb is on top? Left thumb on top = dominant (L-), right thumb on top = recessive (ll).
9. Arm folding: Fold your arms across your chest. Which arm is on top? Left arm on top = dominant (A-), right arm on top = recessive (aa).
1. Fill in the table. If you display the dominant phenotype, be sure to examine other members of your family so you can determine what your genotype must be.
|little finger alignment|
|length of big toe|
2. Choose 2 of your traits from the above table, with at least one of the traits being the dominant phenotype. For each trait, draw a pedigree of your family depicting what each family member’s genotype is and the inheritance pattern of the given trait (see below for proper pedigree notation). Don’t forget to include yourself in each pedigree, and label which one you are. It’s OK if some of your family members have a genotype depicted as “A-” but try to fill in the missing allele if you can. However, if you have the genotype “A-“, the goal is to collect enough information to determine what the unknown allele must be.
Genetics problems: Sex-linked traits
The sex chromosomes (particularly X chromosomes) carry genes that affect traits other than sexual traits. Genes on the sex chromosomes are called sex-linked genes. Because the X chromosome is so much larger than the Y, most sex-linked genes are X-linked.
Red-green color blindness is a recessive X-linked trait. For a quick and easy online colorblind test, click here.
3a. (Answer only if you’re a male.) Are you colorblind? If so, what’s your genotype? If not, what’s your genotype?
3b. (Answer only if you’re a female.) Are you colorblind? If so, what’s your genotype? If not, you can determine whether you’re homozygous dominant (XΒXb) or a carrier for color blindness (heterozygous, XBXb) by knowing if others in your family are colorblind. For example, if your father is colorblind, you automatically know your genotype. What’s your genotype?
4. Answer the following.
a. The only colorblind member of Arlene’s family is her brother. What is her brother’s genotype?
b. What is her father’s genotype?
c. What is her mother’s genotype?
d. What is Arlene’s genotype if she later has a colorblind son?
Experiment: pedigree charts
Pedigree charts show the inheritance pattern of a genetic disorder within a family and can help determine whether a particular individual has an allele for the given disorder. In pedigrees, Roman numerals indicate the generation, circles represent females, and squares represent males. A colored circle or square indicates an afflicted individual.
5. In the pedigree below, is the genetic disorder autosomal dominant, autosomal recessive, or X-linked recessive? For all individuals in the pedigree, give their genotype. (If one allele is known but the other isn’t, use the notation A- if autosomal or XAX- if X-linked.)
6. In the pedigree below, is the genetic disorder autosomal dominant, autosomal recessive, or X-linked recessive? For all individuals in the pedigree, give their genotype. (If one allele is known but the other isn’t, use the notation A- if autosomal or XAX- if X-linked.)
7. In the pedigree below, is the genetic disorder autosomal dominant, autosomal recessive, or X-linked recessive? For all individuals in the pedigree, give their genotype. (If one allele is known the other isn’t, use the notation A- if autosomal or XAX- if X-linked.)