Human Pedigrees

Pedigree Analysis: Human Chromosomes Inheritance patterns are typically studied by tracking observable traits in families over generations. Geneticists study inheritance patterns in humans by tracking genetic disorders and abnormalities through families. Charting genetic connections with pedigrees reveals inheritance patterns of certain traits. A standardized chart of genetic connections (or pedigree) is used to determine the possibility that future offspring will be affected by a genetic abnormality or disorder. Pedigree analyses also reveal whether a trait is associated with a dominant or recessive allele, and whether the allele is located on an autosome or a sex chromosome.

Standard Symbols Used in Pedigrees male female marriage/mating offspring individual showing trait being studied sex not specified generation

Polydactyly

Pedigree Analysis

Hemophilia A in Descendants of Queen Victoria of England Figure 14.7 A classic case of X-linked recessive inheritance: a partial pedigree of the descendants of Queen Victoria of England. At one time, the recessive X-linked allele that resulted in hemophilia was present in eighteen of Victoria’s sixty-nine descendants, who sometimes intermarried. Of the Russian royal family members shown, the mother (Alexandra Czarina Nicolas II) was a carrier. A classic case of X-linked recessive inheritance: a partial pedigree of the descendants of Queen Victoria of England. At one time, the recessive X-linked allele that resulted in hemophilia was present in eighteen of Victoria’s sixty-nine descendants, who sometimes intermarried. Of the Russian royal family members shown, the mother (Alexandra Czarina Nicolas II) was a carrier.

A Pedigree for Huntington’s Disease

Pedigree Analysis Pedigree charts are reconstructions of the inheritance of a single phenotype within several generations of a family. By analyzing a pedigree, it is possible to identify patterns of inheritance. First, you must determine the mode of inheritance. Once you have determined mode of inheritance, genotypes can be identified and predictions of possible phenotype and genotype for individuals of future generations can be made.

Pedigree Analysis Mode of Inheritance Rule of Thumb Examples? autosomal dominant trait cannot hide; unaffected parents CANNOT have affected offspring; both sexes affected equally Huntington disease, Marfan syndrome, achondroplasia autosomal recessive often skips generations; unaffected parents CAN have affected offspring; both sexes affected equally sickle cell anemia, albinism, PKL, cystic fibrous X-linked dominant more females affected than males; affected fathers pass trait to all daughters hypophosphotemic rickets X-linked recessive more males affected than females; carrier mothers pass trait to all sons hemophilia, colorblindness Y-linked no females affected; affected fathers pass trait to all sons retinitis pigmentosa, hairy ears sex-limited traits found in males only or females only chest hair, Adam’s apple sex-influenced male dominant; all sons of affected mothers will be affected female dominant; all daughters of affected fathers will be affected male pattern baldness

Pedigree Analysis

Do your Pedigree Worksheet. STOP! Do your Pedigree Worksheet.

Pedigree Analysis Identify the mode of inheritance. autosomal recessive Appears in both sexes with equal frequency. Trait tends to skip generations. Affected offspring are born equally to unaffected parents. When both parents are heterozygous, approximately ¼ of the offspring will be affected.

Pedigree Analysis Identify the mode of inheritance. X-linked recessive More males than females have the disorder. Trait tends to skip generations (affected sons usually born to unaffected “carrier” mothers) Never passed from father to son. All daughters of affected fathers are carriers. Pedigree Analysis

Pedigree Analysis Identify the mode of inheritance. Y-linked disorder Only males are affected. Passed from father to son. Does not skip generations.

Pedigree Analysis Identify the mode of inheritance. autosomal dominant Appears in both sexes with equal frequency. Does not skip generations. When one parent is heterozygous and the other does not carry the allele, about ½ of the offspring will be affected. Offspring with the disorder have a parent with the disorder.

Pedigree Analysis Identify the mode of inheritance. X-linked dominant Both sexes affected, often more females than males. Does not skip generations. Affected sons must have a mother with the disorder; affected daughters have either a mother or father with the disorder. Affected fathers will pass the trait to all daughters. Heterozygous mothers will pass the trait to ½ of her sons and ½ of her daughters. Pedigree Analysis