Mendelian Genetics

1
Mendelian Genetics
2
From Gene to Trait

• The contributions of Gregor Mendel are referred
  to as Mendelian genetics.
• It involves the basic laws of inheritance and
  some general principles about the relationship
  between the genetic code and the traits that are
  the end product of that code.

3

• The genetic principles described by Mendel form
  the basis of modern genetics.
• Although farmers and herders realized they could
  manipulate the frequency and expression of
  desired traits in plants and animals, no one
  previous to Mendel could explain how these traits
  were affected through selective breeding.
• The predominant belief centered on the blending
  of parental traits in the offspring.
• Even Darwin believed in some aspects of blending
  inheritance, since he was unaware of Mendel's
  work.

4
The Genetic Principles Discovered by Mendel

• Gregor Mendel (1822-1884) developed his theory of
  heredity while working with garden pea hybrids.
• Purebred strains were crossed to produce hybrids,
  and Mendel calculated the frequencies of traits
  in each generation.
• These results were the empirical basis for his
  theory.

5

• Mendel conducted 8 years of extensive breeding
  experiments.
• He crossed plants that exhibited different
  expressions of a trait and then crossed hybrids
  with each other.
• He used only traits that were monogenica trait
  coded for by a single gene.
• He reach the conclusion that each organism
  possess two genes from each traitone from each
  parent.
• Not only does each organism posses two of each
  gene, but genes may come in different versions.
  These are called alleles.
• Alleles variants of a gene.

6
Segregation

• The parental (P) generation was crossed to
  produce the first filial (F1)generation.
• The F1 generation did not have intermediate
  traits.
• The F1 generation was then crossed to produce the
  F2 generation.
• One expression of the trait, shortness of the
  stem or wrinkling of the seeds, for example,
  disappeared in the F1 generation, but reappeared
  in the F2 generation.
• The expression that was present in the F1
  generation occurred more often in the F2
  generation (in a 31 ratio).
• Mendel concluded that discrete units, occurring
  in pairs and separating into different sex cells,
  must control the traits.
• This is Mendel's principle of segregation.

7
Dominance and Recessiveness

• Some alleles are dominate and some are recessive
• Dominate the allele of a pair that is expressed
  in the phenotype.
• Recessive the allele of a pair that is only
  expressed if homozygous.
• Homozygous having two of the same allele in a
  gene pair.
• Heterozygous having tow different alleles in a
  gene pair.
• Dominate alleles are not necessarily better or
  more common.
• They simply mean that if two alleles in the
  relationship are in a heterozygous genotype, the
  action of the dominate will be expressed and the
  action of the recessive will be hidden.

8

• EX PTC a chemical that people can either taste
  or not. For those who can, it has a bitter
  flavor.
• The taster trait is monogenic, but the trait has
  two alleles T, which codes for the ability to
  taste PTC, and t, which codes for the inability
  to taste the chemical.
• Two genes for the taster traitone from the
  father and one from the motherwith three
  possible combinations
• Genotype Phenotype
• TT taster
• tt nontaster
• Tt taster

9
Independent Assortment

• Mendel made crosses with two traits
  simultaneously, such as plant height and seed
  color.
• The results indicated that the proportion of F2
  traits did not affect each other.
• Mendel stated this relationship as the principle
  of independent assortment.
• The loci coding for height and seed color
  happened to be on different chromosomes
  that assort independently of each other during
  meiosis and were therefore not linked.

10
How Inheritance Works

• Genes come in pairs, so to do the chromosomes
  that carry these genes.
• Thus, the members of a pair of genes are found on
  a pair of chromosomes.
• When a cell divides, each chromosome copies
  itself.

11
Mendelian Inheritance in Humans

• Mendelian traits are also called discrete traits
  or traits of simple inheritance.
• There are over 9,600 Mendelian traits in humans.
• Most are biochemical in nature and the result of
  harmful alleles.
• Traits may be inherited either as dominant or
  recessive alleles.Recessive conditions are
  typically associated with the lack of a
  substance.
• Individuals who are heterozygous are termed
  carriers.
• The probability of having an affected child when
  both parents are carriers is 25.
• The ABO blood groups are inherited in a Mendelian
  fashion.
• Dominance, recessiveness, as well as codominance
  are illustrated in this system.

12

• Alleles for most traits do not work this neatly.
• In most cases, heterozygous genotypes result in
  phenotypes that exhibit some action of both
  alleles.
• codominate-- blood types
• codominate when both alleles of a pair are
  expressed in the phenotype.
• Genotype Antigens on Phenotype
• Red blood cells
• AA, AO A A
• BB, BO B B
• AB A and B AB
• OO none O

13
Misconceptions Regarding Dominance and
Recessiveness

• Some traits, such as eye color, are mistakenly
  described as having Mendelian inheritance.
• Eye color is in fact determined by alleles
  occurring at two or three loci.
• Dominance and recessiveness are not
  all-or-nothing situations.
• Recessive alleles may have an effect on the
  phenotype in the heterozygous condition.
• Several alleles are known to have effects on the
  phenotype at the biochemical level.
• Dominant alleles are not "stronger", "better", or
  more common than recessive alleles.

14
Patterns of Inheritance

• Six different modes of Mendelian inheritance have
  been identified in humans through the use of
  pedigree analysis autosomal dominant, autosomal
  recessive, X-linked recessive, X-linked dominant,
  Y-linked, and mitochondrial.
• Autosomal dominant traits are governed by loci on
  the autosomes.
• All affected family members have at least one
  affected parent.
• Males and females are equally affected.
• Autosomal recessive traits are also influenced by
  loci on autosomes.
• Pedigrees for autosomal recessive traits differ
  from those for autosomal dominant traits.
• Recessive traits may appear to skip generations
  if both parents are carriers.
• Most affected individuals have unaffected
  parents.
• The frequency of affected offspring from most
  matings is less than 50.
• As in autosomal dominant traits, males and
  females are equally affected.

15

• Sex-linked traits are affected by loci on either
  the X or Y chromosome.
• Most of the approximately 250 known sex-linked
  traits have loci on the X chromosome.
• Because females have two X chromosomes, they
  have an autosomal-like pattern of expression.
• Males, having only one X chromosome, are
  hemizygous, and cannot express dominance or
  recessiveness for X-linked traits.