Inheritance

Introduction

Patterns of inheritance occur because of the specific genes transmitted through reproduction. These patterns are established based on the Dominance or Recessiveness of a particular trait. The traits or alleles appear on either Sex Chromosomes or Autosomal Chromosomes. The expression of genes for certain diseases may result in the child being a carrier for the condition, or being affected by the disease.

Definitions

Allele: Alternative versions of a gene that produce disinguishable phenotypic effects.

Autosome: A chromosome that is not directly involved in determining sex, as opposed to a sex chromosome.

Chromosome: A threadlike, gene-carrying structure found in the nucleus. Each chromosome consists of one very long DNS molecule and associated ptroteins.

Codominance: The situation in which the phenotypes of both alleles are exhibited in the heterozygote.

DNA - Deoxyribonucleic Acid: A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell’s proteins.

Dominant Allele: An allele that is fully expressed in the phenotype of a heterozygote.

Gene: A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).

Genotype: The genetic makeup, or set of alleles, of an organisnm.

Heterozygous: Having two different alleles for a given gene.

Homozygous: Having two identical alleles for a given gene.

Phenotype: The physical and physiological traits of an organism, which are determined by its genetic makeup.

Recessive Allele: An allele whose phenotypic effect is not observed in a heterozygote.

Sex-Linked Gene: A gene located on a sex chromosome.

Procedure

The pedigree for each of three conditions was observed. Based on the pattern of inheritance, it was determined that the trait was Dominant, Co-dominant or Recessive. By observing the sex of the affected child, it was determined that the trait was Autosomal or Sex-linked.

Discussion

Dominant genes are present in at least one parent in each generation where a condition was passed to the offspring. When an affected parent mates with an unaffected and non-carrier mate; the offspring are either affected or unaffected, but not carriers for the trait.

In codominance, neither phenotype is completely dominant; the heterozygous individual will express both phenotypes.

Parents who were carriers for a particular trait were unaffected by the faulty gene. Recessive genes present were transmitted when the child received the gene from both parents. When a child received only one recessive gene, it resulted in that child being a carrier for that trait.

Autosomal genes are the non-sex determining genes. An autosomal trait can be located on any chromosome but the sex chromosomes.

Sex-linked inheritance occurs when a particular gene is located on the sex chromosomes. X-linked recessive traits are expressed in all male children (or heterogametic = XY); but in female children (or homogametic =XX), only if they inherited the gene from both parents.

X-linked recessive inheritance occurs more frequently in males because they have only one X chromosome, from the mother. Female children, if they inherit one copy of the gene, will be carriers for the trait. If the affected male has a daughter, he will pass on the gene. Since the father contributes the Y chromosome to his sons, the sons will not inherit the gene from their father. Many who inherit X-linked disorders will not survive to reproduce.

X-linked dominant traits are often lethal to male offspring and are less common than X-linked recessive genes. Women with an X-linked dominant disorder have a 50% chance of passing the trait to any children. Men with an X-linked dominant disorder will not pass the gene to his sons, but all his daughters will inherit the condition.

Y-linked traits produced by a gene called the SRY or Sex-determining Region Y. These genes often cause ambiguous genitalia, and it is not immediately apparent whether the child is genetically male or female.

With an autosomal dominant gene, the phenotype will be expressed, even if the recipient is heterozygous for the trait.

Autosomal recessive genes, on the other hand, must be homozygous, or inherited from both parents, for the trait to be expressed. If both parents are heterozygous for the trait, there is a 25% chance that the child will have the trait; 50% of the children will be carriers.

The three conditions chosen were Huntington’s Disease, Muscular Dystrophy and Albinism.

Huntington’s Disease

Huntington’s disease is an inherited neurological disorder that affects approximately 10 persons in 100,000. Dr. George Huntington first described it in 1872. The disease produces an altered form of a protein that causes cell death in certain areas of the brain. Huntington’s disease is progressive and terminal. The physical symptoms include abnormal movement and lack of coordination, as well as changes in mental abilities and personality. There is no know cure, but symptoms can be managed for a time with medication and therapy.

Figure 1 displays the pedigree for a family affected by Huntington’s Disease. The condition is a Dominant trait found on Autosomal genes. The trait is dominant because when a parent was affected, children that inherited the gene were also affected. Dominant traits do not skip generations. Each of the three generations has affected individuals. The first two affected parents were both female. In both the second and third generations of this family, both male and female offspring were affected. Therefore, this condition