5.4 Non mendelian genetics

Unit 5.4: Non-Mendelian Genetics

Gene Linkage and Mapping

Gene linkage refers to the phenomenon where genes located close together on the same chromosome tend to be inherited together. This section explores the principles of linkage, recombination, and genetic mapping.

• Definition: Genes located close together on the same chromosome that tend to be inherited as a unit.

• Physical Proximity: The closer two genes are, the less likely they are to be separated during crossing over in meiosis.

• Deviation from Mendel: These do not produce the predicted 9:3:3:1 dihybrid ratios and are observed with linked genes.

• Recombination Frequency (RF):

  • Linked Genes: Always have an RF of less than 50%.

  • Unlinked Genes: RF of 50% indicates genes are on different chromosomes or far apart.

• Mapping: The probability of recombination is used to calculate map distance.

  • 1 map unit (centimorgan) = 1% recombination frequency.

  • Example: If two genes are 15 map units apart, they are 15% likely to be separated by crossing over in meiosis.

Sex-Linked Inheritance

Sex-linked traits are determined by genes on sex chromosomes, most commonly the X chromosome. These traits often display unique inheritance patterns due to the difference in chromosome composition between males and females.

• Location: Traits determined by genes on sex chromosomes (usually the X).

• Hemizygosity: Males (XY) are hemizygous, expressing any recessive X-linked allele they inherit.

• Example: Red-green color blindness is X-linked recessive. A male only needs one copy of the allele to express the trait.

• X-Inactivation: In females, one X chromosome is randomly inactivated in each cell, forming a Barr body.

• Manifestation: This can create mosaic phenotypes (e.g., tortoiseshell cats).

Non-Nuclear Inheritance

Some genes are found outside the nucleus, in organelles such as mitochondria and chloroplasts. These genes are inherited independently of nuclear chromosomes.

• Location: DNA found in mitochondria and chloroplasts, inherited solely through the maternal line.

• Inheritance: Offspring inherit these traits from their mother only.

• Example: Mitochondrial diseases are passed from mother to all offspring, but not via fathers.

Polygenic Inheritance & Epistasis

Some traits are controlled by multiple genes (polygenic), while epistasis occurs when one gene affects the expression of another gene.

• Polygenic Inheritance: A single trait is governed by many genes, resulting in continuous variation (e.g., skin color, height).

• Epistasis: One gene modifies or masks the effect of another gene at a second locus.

  • Example: In Labrador retrievers, coat color is influenced by two genes: one determines pigment color (B = black, b = brown), and another determines pigment deposition (E = pigment, e = no pigment).

Environmental Impacts (Phenotypic Plasticity)

Phenotypic plasticity refers to the ability of a genotype to produce different phenotypes in response to environmental conditions.

• Definition: Non-genetic factors can influence the phenotype produced by a single genotype under different environmental conditions.

• Example: The coat color of Himalayan rabbits changes with temperature; cooler areas of the body become darker.

• Epigenetics: Chemical modifications (such as DNA methylation) can alter gene expression without changing the DNA sequence itself.

Statistical Analysis (Chi-Square Test)

The chi-square test is used to determine if observed data significantly deviates from expected Mendelian ratios, helping to assess genetic hypotheses.

• Purpose: To determine if observed data significantly deviates from expected Mendelian ratios.

• Decision Rule:

  • If $\chi^2 >$ Critical Value (at $p=0.05$): Reject the null hypothesis. The data does not fit Mendel's laws and is likely influenced by linkage or other factors.

  • If $\chi^2 \leq$ Critical Value: Fail to reject the null hypothesis. Any variation is likely due to random chance.