Patterns of Inheritance

• What are some characteristics controlled by a single gene?

  Fur colour in mice and red-green colour blindness
• What are different forms of a gene called?

  Alleles
• What are the two alleles for the attachment of earlobes?

  Attached or free
• How many alleles does an individual have for each gene?

  Two alleles
• Where does one allele for a gene come from?

  From the mother
• Where does the other allele for a gene come from?

  From the father
• What is the term for the combination of alleles present in a gene?

  Genotype
• How is the genotype represented?

  By two letters, one for each allele
• What does phenotype refer to?

  How the alleles are expressed
• What are the two types of alleles?

  Dominant and recessive
• What does it mean if a person is homozygous for a gene?

  The two alleles present are the same
• What does it mean if a person is heterozygous for a gene?

  The two alleles present are different
• Key Point

  A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present, i.e. no dominant allele is present.
• What type of inheritance involves several genes working together?

  Polygenic inheritance
• What is it called when only one gene is involved in inheritance?

  Monohybrid inheritance
• How can genetic diagrams predict the outcome of a monohybrid cross?

  By using Punnett squares
• What do lower case letters represent in genetic diagrams?

  Recessive alleles
• What allele represents a free lobe in earlobes?

  E
• What allele represents an attached lobe in earlobes?


  e
• What are the possible outcomes of three crosses in a Punnett square for earlobes?

  • 1 in 2 chance for attached lobes
  • 100% chance for free lobes
  • 3 in 4 chance for free lobes
• Why does each offspring have a 1 in 2 chance of having attached lobes?

  Because the dominant allele is present in half the crosses
• Why does each offspring have a 3 in 4 chance of having free lobes?

  Because the dominant allele is present in three crosses
• What is the phenotype for free lobes when the dominant allele is present?

  Free lobes
• Inherited Disorders

  . Some human disorders are inherited and are caused by the inheritance of certain alleles:

  - Polydactyly (having extra fingers or toes) is caused by a dominant allele. Only one copy of the dominant allele is needed for the polydactyly phenotype.

  . Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele. Both copies of the recessive allele are needed for the cystic fibrosis phenotype

  . There are always risks in having embryonic screening tests. The test could also be a false positive or negative
• Key Point

  It is now possible to test unborn foetuses for a range of genetic disorders. However, this means that the parents may have to make difficult decisions about the future of their baby. Options could include terminating the pregnancy, or undergoing gene therapy. Gene therapy may ease suffering but there may be ethical issues that need to be considered. Decisions made need to take into account the risks and benefits of all decisions based on the evidence and arguments.
• Sex Determination

  . Only one pair out of the 23 pairs of chromosomes in the human body carries the genes that determine sex

  . These are called the sex chromosomes

  . In females, the two sex chromosomes are identical and are called X chromosomes (XX)

  . Males inherit an X chromosome and a much shorter chromosome called a Y chromosome (XY)

  . As with all chromosomes, offspring inherit:

  - one sex chromosome from the mother (X)

  - one sex chromosome from the father (X or Y)