BIO 208 Genetics 2010 1 Applied Human Genetics Pedigree Analysis Monohybrid Cross Dihybrid Cross Chi Square Analysis Probability Epistasis I. Applied Human Genetics/Single Gene Traits The classical study of single gene traits is by pedigree analysis. Over 7000 human single gene traits have been recognized, many also assigned to the particular chromosome on which they reside. The Human Genome Project will uncover many more genes and further study in proteomics will assign function to these genes. For more information on the mechanism of inheritance of single gene traits, allelic variations, and chromosome mapping, consult Online Mendelian in Man www.OMIM.gov. The genotype of a homozygous recessive individual is usually known by the phenotype exhibited. For example, individuals with blue eyes are bb. However, the dominant phenotype could arise from two genotypes. A person with the dominant brown eye phenotype could have the BB or Bb genotype. Therefore the genotype is represented as B-. The dash indicates that while a second allele is present, the nature of that allele is unknown BB = homozygous dominant Bb = heterozygous B- indicates dominant phenotype bb = homozygous recessive Single gene traits Interlocking fingers (I) Interlock your fingers and observe which thumb is on top (right or left). The tendency to place the left thumb on top is due to a dominant allele (I). Those who place the left thumb on top have the genotype I- (either II or I-). The right thumb on top is determined by the ii genotype Ear lobes (E) Examine your partner’s earlobes. The dominant allele (E) results in the phenotype of free earlobes. The recessive allele (e) is for attached earlobes. Earlobes are attached if the bottom lobe is attached directly to the head. Earlobes are free if the lobe hangs free. Widow’s peak (W) Widow’s peak occurs when the hairline forms a distinct point in the center of the forehead. Lift up the hair of your forehead to score this trait. People without widow's peak have a smooth hairline with no dip. Men starting to go bald (or already bald) may be unable to score this trait. Widow's peak is controlled by a dominant allele (W) Tongue curling (T) A dominant allele (T) results in the ability to curl the tongue in a U-shape Hitch hiker’s thumb (H) A person homozygous recessive for this trait (hh) can bend the last (distal) thumb joint back to about a 90 degree angle. Those with the H allele cannot Pigmented iris (B) A person with the dominant allele has brown or brownish eyes. The recessive allele encodes blue eyesBIO 208 Genetics 2010 2 Polydactyly (PD) A person with polydactyly has >5 fingers on 1 or more hands and feet. It is dominant (PD) Mid-digital finger hair Examine middle segment of your fingers. If hair is present, even one, you have mid-digital hair. PTC tasting (P) If you can taste PTC, you have a dominant allele for this trait (P). Place the strip of PTC paper on your tongue for a few seconds. If you cannot taste anything, you do not possess the dominant allele Fill in the following table Gene Describe phenotype Genotype Earlobes (E) Widows peak (W) Tongue roll (R) Hitchhikers thumb (H) Pigmented iris (P) Polydactyly (PD) PTC taster (T) Interlocking fingers (I) Class phenotypic frequencies. Gene f (E-) dominant phenotype f(ee) recessive phenotype Tongue roll (R) Pigmented iris (P) Polydactyly (PD) PTC taster (T) Interlocking fingers (I)BIO 208 Genetics 2010 3 II. Pedigree Analysis Symbols 1. A man who has pointed ears came to the attention of a geneticist. The geneticist finds the following: Pointy ears prove to be an inherited trait due to a single genetic locus. The man's one sister also has pointed ears, but his mother, father, his brother, and other sister have normal ears. The man and his normal-eared wife are first cousins and have seven children, including four boys and three girls. Two girls and two boys have pointy ears, the two boys are identical twins. (a) Draw the pedigree (b) Which one of the following best describes the inheritance of pointy ears in this family? Autosomal recessive Autosomal dominant Sex-linkedBIO 208 Genetics 2010 4 2. Asparagus officinalis (asparagus) is a member of the lily family and a popular vegetable to grow and eat. A pungent urinary odor produced within a few hours of eating asparagus is due to sulfur-containing metabolic breakdown products, or S-methyl thioesters. The smell cannot be noticed in raw or cooked asparagus. The odor is described as that of boiled cabbage or ammonia. About 40% of the population can detect S-methyl thioesters. All people produce the thioesters, it is the ability to smell them that is inherited. Those who are able to detect the odor in their own urine can smell it in the urine of anyone who has eaten asparagus. Pedigree of the ability to detect S-methyl thioesters (a) Is the ability to detect S-methyl thioesters inherited as a dominant, or as a recessive, trait? How do you know (which individuals inform you about the inheritance)? (b) Label the genotypes of all individuals on the pedigree (c) Explain (or diagram) how individuals III,1 and III, 6 could have a non-smeller child I II III IVBIO 208 Genetics 2010 5 3. Alkaptonuria is a rare genetic disease in which the body does not have enough of the enzyme homogentisic acid oxidase (HGAO). Because normal amounts of the HGAO enzyme are missing, homogentisic acid (HGA) is not used and builds up in the body. Some is eliminated in the urine and if left standing for several hours will turn brownish black. The rest of the pigmented material is deposited in body tissues where it is toxic. Patients with alkaptonuria are usually not aware of the disease until about age 40 when symptoms are present. Dark staining of the diapers sometimes can indicate the disease in infants, but usually no symptoms are present until much later in life. Alkaptonuria affects many body systems: Skeletal – knees and hips most affected. Deposits of pigment cause brittle cartilage Cardiovascular - The aortic and mitral heart valves are most affected. Pigment (orchronotic) deposits also can lead to the formation of atherosclerotic plaques Genitourinary - Pigment deposits can form stones in the prostate. Respiratory - Pigment deposits in the cartilage of the larynx, trachea, and bronchi. Ocular - Vision is not usually affected, but