Chapter 16 Summary

Mendel's Law of Experiment

Mendel’s experiments have led to the development of basic genetic concepts, known as Mendel’s laws. Mendel began his experiment by hybridizing pea plants and observing its phenotype (physical appearance). There were many pluses to his experiment because the nature of pea plants could easily be manipulated and crossed in many ways. Mendel started the experiment with a monohybrid cross in which two genetically homozygous plants were crossed, one carried the gene TT, and the other carried the gene tt. The result of a monohybrid cross is always a heterozygous dominant gene, Tt. He then crossed the Tt genes together. This showed the nature of dominant and recessive traits. Each gene has two alleles, one which is expressed, and one which isn’t.  A dominant trait is the one expressed in phenotype. In a dominant gene, of the two alleles, either one or both have to dominant. The presence of only one recessive allele doesn’t hinder the expression of a dominant gene. Recessive genes are only expressed when there are two recessive alleles present. This led to the development of the law of segregation: during meiosis, no gamete receives more than one allele for a particular gene; each gene segregates during gamete formation.

Mendel also determined how we could use crossing to predict a genotype. If the gene for a dominant trait is not known, it can be crossed with the recessive phenotype of that gene. If all generations show the dominant gene, then the gene is homozygous dominant. If only half show the dominant trait and the other half show a recessive trait, then the gene is heterozygous dominant.

In a dihybrid cross, two genes are crossed at once. Looking at a gene, let’s say TtGg, first, all the possible gametes are recorded. In this case, the gametes would be: TG, Tg, tG, and tg (keeping in mind law of segregation). When these four gametes are crossed with gametes with the same genotype, the phenotypic ratio is 9:3:3:1 in which 9 are dominant for both genes, 3 are dominant for the first and recessive for the second, 3 are recessive for the first and dominant for the second, and 1 is recessive for both. This led to the law of independent assortment, in which the alleles of different genes assort independently of each other during gamete formation. 

The Chromosome Theory of Inheritance

 The five main principles of the of the chromosome theory of inheritance are

1. Chromosomes contain DNA which contain the genes. 
2. chromosomes are replicated and passed from parent to offspring and from cell to cell during development. 
3. the nucleus has a diploid set, homologous pairs, of chromosomes. One set is maternal and the other is paternal. Each set has a complete set of genetic information. 
4. During meiosis, homologous segregate into separate daughter nuclei. During gamete formation, each sister chromatid of a chromosome pair segregates independently. 
5. Gametes are haploid and combine to form diploid cells during fertilization. Each gamete contributes a complete haploid set of chromosomes.  

This inheritance theory was closely related to Mendel’s laws of inheritance. Mendel’s law of segregation explains the independent assortment of genes during meiosis. In meiosis I, homologues line up independently along the center, with no all maternal on one side and all paternal of the other. After the homologues separate, each individual chromosome lines randomly and the sister chromatids once again separate independently. If this weren’t to occur, there would be no diversity in our genes because the gametes would then me all maternal genes or all paternal genes. And because each homologue contains a variant of the same gene at each locus (specific gene location), each gamete contains a full set of genetic information. 

Pedigree Analysis of Human Traits

 In a  pedigree analysis, the inheritance of a particular trait is traced through several generations. Pedigrees allow us to determine whether the trait is dominant, recessive, of sex-linked and the likelihood of a person inheriting that trait. In the pedigree of a recessive trait, there can be three different types of symbols: affected individuals contain filled shapes (female – circle; male – square), unaffected individual contain empty shapes, and carriers contain half filled shapes. Because the trait is recessive, we can have carriers, since the genotype of a recessive trait is homozygous. In a pedigree of a dominant trait, the shapes are either filled in or empty. There can be no carriers because a dominant trait is expressed with the presence of even one dominant gene. Dominant and recessive pedigrees are based of autosomal traits, in which the gender plays no role. 

Sex Chromosomes and X-Linked Inheritance Patterns

Sex linked traits depend on gender. While some traits are only found in males, other traits can be found only in females. But first we must determine the genes that differentiate a male and a female. In humans the genes are X and Y. XX means females and XY means a male. The mother always contributes an X, the father can contribute either an X or a Y. Insects follow a X-O system in which the ratio of X chromosomes to the autosomal genes determines gender. If the ratio is one X to a diploid autosome, the insect will be male; if the ratio is two X’s to a diploid autosome, the insect will be female. In birds, the male is homozygous with ZZ, and female is heterozygous with ZW. In bees, the fertilized gamete becomes female (diploid) and the unfertilized gamete becomes male (haploid).

In humans, the X is the larger chromosome, thus contains a more genes and more traits. These traits are called X-linked genes. Males are hemizygous, meaning they contain only one X chromosome. This makes males more likely to have certain traits because females have an extra X to “cover for” the other. 

Variants in Inheritance Patterns and Their Molecular Basis

Mendel’s inheritance can be described in five different ways.  The first is Simple Mendelian Inheritance, in which the pattern of traits is determined the dominant/recessive relationship of an autosome. The recessive gene is nonfunctional when in presence of a dominant gene. The second type is X-linked Inheritance, in which the dominant/recessive relationship is observed on X chromosomes. Because males are hemizygous, they exhibit these traits more frequently.  The third type is Incomplete Dominance, in which the phenotype of a heterozygote is similar to phenotype of the crossed homozygotes. 50% of the gene is not enough to produce enough proteins to express the complete trait. The fourth type is Codominance, in which the phenotypes express both alleles simultaneously. The alleles of the gene encode proteins slightly different, leading to the expression of both. The last type in Sex-Influences Inheritance, in which alleles is dominant in one sex but recessive in the other. Sex hormones affect the expression of genes. 

Genetics and Probablity

Useful Materials

1. Huntington's Disease - This talks about the development of Huntington's disease. There is no cure for it and its effects cannot be slowed. DEath usually occcur 20 years after diagnosis. 

2. This video was useful because it explains how dihyrid crosses are done and the concept of gamete formation for the crosses.

3. This image pretty much sums up the importance of alignment and segregation during gamete formation.