6.1.2 Patterns of Inheritance

• Define what is meant by a gene

  • Gene - Sequence of DNA bases that code for a polypeptide, resulting in a characteristic
• Define what is meant by genotype

  Genotype - an organisms genetic makeup
• Define what is meant by phenotype

  Phenotype - an organism's physical characteristics as determined by its genotype and environment
• Define what is meant by dominant

  Dominant - an allele that is always expressed in the phenotype even when only one copy is present in the genotype
• Define what is meant by recessive

  Recessive - an allele that is only expressed when it is homozygous in the genotype (2 copies present) and is masked by a dominant allele
• Define what is meant by a locus

  Locus - the specific position of a gene on a chromosome
• Define what is meant by homozygous

  Homozygous - an organism with two identical alleles for a trait
• Define what is meant by heterozygous

  Heterozygous - an organism with two different alleles for a trait
• Why are gametes haploid?

  • They have one allele per gene
• What is monohybrid inheritance?

  • Transmission of one gene from parents to their offspring
• What is the F1 generation?

  • Offspring from a cross between individuals with homozygous dominant and homozygous recessive genotypes
  • All offspring will be heterozygous, inheriting recessive allele from one parent and dominant from the other
  • Offspring in the F1 generation will express the dominant trait in their phenotype

  
• What is the F2 generation?

  • Offspring from a cross between 2 heterozygous F1 individuals
  • Results in offspring exhibiting a 3:1 ratio of dominant to recessive phenotypes

  
• What is continuous variation?

  • Quantitative
  • Can be measured
  • Controlled by genes and the environment
  • Range of phenotypes between 2 extremes
• What is discontinuous variation?

  • Qualitative
  • Can't be measured
  • Controlled by genes (only a few)
  • Few phenotypes
• What factors affect phenotypic variation?

  Genetic:

  • Polygenic - controlled by many genes (continuous)
  • Monogenic - controlled by one gene (discontinuous)

  Environmental:

  • Etiolation - plants grow abnormally long and spindly due to not enough light
  • Chlorosis - plants do not produce enough chlorophyll due to lack of magnesium
• Define codominance

  Codominance - 2 different alleles are expressed in an organism’s phenotype as the alleles have equal dominance
• How is codominance shown in genetic diagrams?

  • Uppercase for the gene
  • Superscript uppercase for alleles
  • E.g. CR and CW
• What are autosomes?

  Autosomes - non sex chromosomes

  • 22 pairs
  • When genes are on the same autosome they are linked genes that tend to be inherited together as they stay together during independent assortment
• What are sex-linked genes?

  Sex-linked genes - genes located on the X or Y chromosomes

  • X chromosome carries the majority of these genes due to its larger sized so most sex-linked genes are X linked
  • Recessive alleles on the X chromosome tend to appear in the phenotype more often in males as no corresponding allele on Y to mask them
• Describe sex-linkage and inheritance of Haemophilia

  • Recessive X linked allele that alters the DNA sequence coding for crucial blood clotting protein causing reduced blood clotting and excessive bleeding
  • Inherited even if both parents don't show symptoms e.g. mother is a carrier

  Inheritance:

  • Mainly affects males
  • Doesn't often affect females
  • Always inherited from mother in males
  • Mainly inherited from carrier mothers
  • Affected fathers can only pass it on to daughters (sons only inherit Y from father)

  
• What are multiple alleles?

  Multiple alleles - genes that exist in more than 2 allelic forms

  • An individual can only have 2 alleles of a specific gene at any one time and these alleles often encode slightly different proteins
• What is a dihybrid cross?

  Dihybrid cross - shows the simultaneous inheritance of 2 genes controlling separate characteristics
• What are the uses of dihybrid crosses?

  • Determine whether genes are linked
  • Locate genes on specific chromosomes
  • Calculate expected phenotypic ratios in subsequent generations
• What is the typical phenotypic ratio for a dihybrid cross?

  • 9 : 3 : 3 : 1
• Why do dihybrid crosses deviate from expected ratios?

  Random fertilisation:

  • Gamete fusion is a chance process

  Linked genes:

  • Linked genes on the same chromosome so the alleles are usually inherited together
  • Crossing over during meiosis can sometimes change allele combinations by separating linked genes
• Describe the effects of crossing over on autosomal linkage?

  • Crossing over can separate linked genes
  • Less genetic variation from crossing over when genes are linked
  • Genes that are closer together are more likely to be inherited together
• What is epistasis?

  Epistasis - interaction between genes where one gene affects or masks the expression of another gene

  • Occurs when genes on different chromosomes interact to influence the same phenotype
• What is a hypostatic gene?

  Hypostatic gene - gene whose expression is blocked by another gene
• When does recessive epistasis and dominant epistasis occur?

  Recessive epistasis:

  • Occurs when epistatic gene must be homozygous recessive to block the expression of the hypostatic gene

  Dominant epistasis:

  • Occurs when the epistatic gene is dominant and actively modifies or blocks the expression of the hypostatic gene
• What is the typical phenotypic ratio for recessive epistasis?

  9 : 3 : 4
• What is the typical phenotypic ratio for dominant epistasis?

  12 : 3 : 1
• What is the gene pool?

  Gene pool - complete set of genetic information in a population
• Name factors that influence evolution

  • Mutations - new alleles and genetic varation
  • Gene flow - transfer of alleles between populations
  • Genetic drift - random changes in allele frequencies
  • Natural selection - increase frequency of alleles that improve survival and reproductive success
  • Sexual selection - increase frequency of alleles enhancing reproductive success
• Explain how population genetics and natural selection affect evolution

  • Population genetics = variation in allele frequencies over time
  • Size of a population influences genetic variation determining potential of natural selection

  Large population:

  • Large gene pools so high genetic diversity so can adapt effectively to changing selection pressures through natural selection

  Small population:

  • Small gene pools so low genetic diversity so at risk of extinction when changes in selection pressures
• Explain how genetic bottlenecks affect evolution

  • Population size reduces suddenly and drastically and reduction lasts for at least one generation

  Impacts:

  • Reduced gene pool and decreased genetic diversity which causes issues related to inbreeding and reduced fertility
  • However may allow beneficial mutation to become more prevalent
• Explain how the founder effect affects evolution

  • Small group splits from larger population and small new population established

  Impacts:

  • Reduced gene pool and decreased genetic diversity compared to the original population
  • Rare alleles from the original population may become more common in new population
• Explain how variation drives selection

  • Generates range of phenotypes within a population so some individuals more likely have advantageous alleles
  • Individuals with advantageous alleles survive and reproduce under changing conditions passing on advantageous alleles to offspring
  • Natural selection occurs
• Explain how directional selection affects allele frequency

  • Selects for one extreme phenotype over other phenotypes
  • Increases allele frequency for one extreme phenotype
  • Distribution curve shifts in direction of favoured extreme

  E.g. Antibiotic resistance in bacteria

  
• Explain how stabilising selection affects allele frequency

  • Selects for average phenotype and selects against extreme phenotypes
  • Increases allele frequency for average phenotype and decreases allele frequency for extremes
  • Narrows the distribution curve

  E.g. Human birth weights

  
• Explain the effect of disruptive selection on allele frequency

  • Selects for extreme phenotypes and selects against the intermediate phenotype, especially when an environmental factor takes 2 or more distinct forms
  • Increases allele frequency for multiple extreme phenotypes and decrease allele frequency for intermediates
  • Curve shift into multiple peaks either side of where average phenotype was

  E.g. bird beaks adapting to be larger and smaller when 2 different food sources