Genetics & Inheritance

• Chromatin


  Long tangled thread-like structure in the nucleus of an inactive cell made up of DNA
• Chromosomes


  A thread-like structure made up of DNA / that carries hereditary information in the form of genes
• Genes


  A segment of DNA/a chromosome that codes for a particular characteristic
• Alleles


  Two or more versions/ forms of a gene which are located at the same position, or genetic locus, on a chromosome
• Dominant allele


  An allele that masks or suppresses the expression of the allele partner on the chromosome pair and the dominant characteristic is seen in the homozygous (e.g.: TT) and heterozygous state (e.g.: Tt) in the phenotype
• Recessive allele


  An allele that is suppressed when the allele partner is dominant. The recessive trait will only be expressed/seen if both alleles for the trait are homozygous recessive
• Phenotype


  The observable characteristics (physical appearance) or traits of an organism that are produced by the interaction of the genotype and the environment : the physical expression of one or more genes
• Genotype


  The genetic makeup of an organism
• Homozygous


  When two alleles that control a single trait (on the same locus) are identical
• Heterozygous


  When two alleles that control a single trait (on the same locus) are different
• Monohybrid cross


  One characteristic is investigated, so the individuals genotype will consist of two letters e.g. RR or Rr or rr. Gametes will have one letter e.g. R or r
• Complete dominance


  One allele masks the expression of the other allele, e.g. B is dominant over b
• Incomplete dominance


  Neither of the alleles are dominant over each other. An intermediate phenotype (form of the gene) is obtained when both alleles are present. e.g. in flowers RW in the genotype is expressed as pink in the phenotype
• Co-dominance


  Both alleles are equally dominant, and both are expressed in the phenotype e.g. in blood the alleles IA and IB result in the blood group AB
• Sex-linked


  The allele causing the disorder is found on the X-chromosome e.g. XH Xh or Xh Y

  
• Dihybrid cross


  Two characteristics are investigated, so the individuals genotype will consist of four letters e.g. RrYy
• Use genetic crosses to show determination of sex-linked disorders


  1. Identify the letters used to indicate the recessive and dominant alleles

  2. Construct a genetic cross

  3. Determine the possible genotype and phenotype of the children

  
• Use pedigree diagrams to answer questions

  1. Study any key and opening statement/s

  2. Fill in the genotype of all the individuals with the recessive condition

  3. For every individual in the diagram that has the recessive condition, it means that they must have inherited one recessive allele from each parent

  4. Determine the genotype of the parents based on the offspring phenotypes
• Dihybrid cross


  Two characteristics are investigated and therefore there will be four letters in the individual's genotype, e.g. RRYy (two for each characteristics)<|>Gametes will have two different letters e.g. Ry

  
• Gregor Mendel, an Austrian monk, is regarded as the father of genetics for his work on garden pea plants that helped explain how genes are passed from parents to offspring
• Mendel's first Law of Inheritance: Law (principle) of Segregation


  An organism possesses two 'factors' which separate or segregate so that each gamete contains only one of these 'factors'
• Mendel's Second Law of Inheritance: Law of Dominance


  When two homozygous organisms with contrasting characteristics are crossed, all the individuals of the F1 generation will display the dominant trait. An individual that is heterozygous for a particular characteristic will have the dominant trait as the phenotype
• Mendel's Third Law of Inheritance: Law (principle) of Independent Assortment


  The various 'factors' controlling the different characteristics are separate entities, not influencing each other in any way, and sorting themselves out independently during gamete formation
• Format of a genetic cross

  1. P1 and F1

  2. Meiosis and fertilization
• The maximum marks for a genetic cross is (6) so you need to score 6 out of 7 possible marks
• You may also be asked to work out the ratio or % chance of the various phenotypes or genotypes occurring
• Monohybrid crosses: Complete dominance


  One allele masks the expression of the other allele, e.g. in pea plants the allele for green pod (G) is dominant over the allele for yellow pod (g)
• Monohybrid crosses: Incomplete dominance


  Neither of the alleles are dominant over each other. An intermediate phenotype (form of the gene) is obtained when both alleles are present, e.g. in flowers neither the allele for red colour (R) nor white colour (W) is dominant, so the offspring in the F1 generation will have an intermediate or third form of colour – pink
• Monohybrid crosses: Co-dominance


  Both alleles are equally dominant, and both are expressed in the phenotype e.g. in cows the allele for red colour (R) and the allele for white colour (W) are equally dominant, so the offspring in the F1 generation will be red and white in colour

  
• Sex determination


  22 pairs of chromosomes in humans are autosomes<|>1 pair of chromosomes are sex chromosomes or gonosomes<|>Males have XY chromosomes and females have XX chromosomes at pair 23

  
• Genetic cross to show the inheritance of sex

  1. Identify the genotypes of the parents

  2. Determine the possible gametes

  3. Show the genotypes and phenotypes of the offspring

  
• Sex-linked inheritance


  A genetic disorder caused by or linked to gene(s) located in the sex chromosome<|>In humans, the sex chromosomes are the X chromosome and Y chromosome<|>A female individual possesses two X chromosomes whereas a male has X chromosome and Y chromosome<|>The X chromosome carries more genes that are not found in the Y chromosome, so the X chromosome is more commonly linked to genetic mutations and disorders<|>X-linked traits and disorders are expressed more in males than in females because the males have only one copy of the X chromosome

  
• Haemophilia is a sex-linked disease caused by the presence of a recessive allele (Xh)
• Genetic cross to show the inheritance of haemophilia

  1. Identify the genotypes of the parents

  2. Determine the possible gametes

  3. Show the genotypes and phenotypes of the offspring
• Blood groups


  The inheritance of blood groups is an example of multiple alleles<|>The notation for alleles indicating blood groups is only IA; IB and i<|>Different combinations of the alleles result in four blood groups<|>The inheritance of blood groups displays both co-dominance and complete dominance
• The 1-2-3-4 Rule of blood: 1. An individual has ONE blood group, 2. An individual has TWO alleles for their blood group, 3. There are THREE different alleles controlling blood groups, 4. There are FOUR blood groups
• Genetic cross to show the inheritance of blood groups

  1. Identify the genotypes of the parents

  2. Determine the possible gametes

  3. Show the genotypes and phenotypes of the offspring
• Dihybrid crosses


  Involve two pairs of alleles representing two different characteristics<|>According to the Law of Independent Assortment, alleles of different genes move (segregate) independently of each other into the gamete
• Steps to solve a dihybrid cross

  1. Identify the phenotypes of the two organisms for each of the two characteristics

  2. Choose letters to represent the alleles for the gene responsible for each characteristic

  3. Write the genotype of each parent

  4. Determine the possible gametes that each parent can produce

  5. Enter possible gametes on the side and top of the Punnet square

  6. Determine the genotypes of the offspring that will result from each possible combination of gametes

  7. Determine the phenotypes of the offspring from the genotypes

  
• Pedigree diagrams/genetic lineages trace the inheritance of characteristics over many generations