Lecture 12: Multiple Alleles

Diploid and Haploid States

  • Diploid state refers to cells that have two sets of chromosomes (2n), one set from each parent.

  • Gametes (sperm and egg cells) are haploid (n), meaning they contain only one set of chromosomes.

  • When gametes come together during fertilization, they form a diploid organism.

Chromosomes and Chromatids

  • Homologous chromosomes are pairs of chromosomes that have the same genes but may carry different alleles.

  • Sister chromatids are identical copies of a single chromosome formed during DNA replication; they are linked at the centromere.

  • During meiosis, homologous chromosomes segregate, with each gamete receiving one chromosome from each pair.

Genetics Basics

  • Mendel's experiments with pea plants led to the understanding of inherited traits through genes.

  • The appearance of organisms is called the phenotype, which is determined by the combination of alleles they possess.

  • For example, in Mendelian inheritance:

    • Big G / Big G = Green phenotype

    • Big G / little g = Green phenotype

    • Little g / little g = Yellow phenotype (only occurs if homozygous recessive).

Dominance and Codominance

  • Dominance refers to the phenomenon where one allele masks the effect of another in heterozygous conditions.

  • Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits.

Lethal Alleles

  • Alleles can also interact leading to lethal outcomes, where certain genotypic combinations (e.g., homozygous dominant) result in death.

  • This can lead to ratios like 2:1 in offspring (indicating lethal combinations that do not survive).

Multiple Alleles

  • Some traits are governed by multiple alleles, such as ABO blood types, which showcase more than two variations in a population.

    • Example alleles:

      • A (IA) produces A antigens,

      • B (IB) produces B antigens,

      • O (i) produces neither.

    • The presence of A and B antigens on red blood cells can lead to multiple blood types: A, B, AB, and O.

Blood Types and Antibodies

  • Blood type compatibility is crucial due to the presence of antibodies that attack non-self antigens:

    • Type A produces anti-B antibodies,

    • Type B produces anti-A antibodies.

    • Type AB produces neither and can accept any type.

    • Type O is the universal donor and does not produce A or B antigens.

RH Factor

  • The Rh factor (specifically D antigen) is important for blood typing and pregnancy.

  • Rh positive is dominant over Rh negative. If an Rh-negative mother carries an Rh-positive baby, there could be complications like hemolytic disease of the newborn (HDN).

Inheritance Patterns

  • Important genetic concepts include:

    • Homozygous (both alleles the same) vs. Heterozygous (two different alleles).

    • How combinations of alleles affect overall phenotype and health.

Genetic Probability Problems

  • Inheritance can be calculated using Punnett squares or probability rules.

  • Example problem: if both parents are heterozygous for blood type, calculate the probability of offspring exhibiting type A.

Key Takeaways for Exam

  • Understand the roles of haploid and diploid states in genetics and reproduction.

  • Grasp the distinction between homologous chromosomes and sister chromatids.

  • Be able to identify and explain simple dominance, codominance, and other inheritance patterns.

  • Recognize the significance of blood type incompatibilities and the implications of RH factor.