Chromosomal Variation in Sexual Reproduction (College Board AP Biology)

This component of the AP course is repeated in more than one subtopic in the syllabus

Below are links to the alternative location of the specification points shown

The AP Course Description contains the following points for this topic:

• "Segregation, independent assortment of chromosomes, and fertilization result in genetic variation in populations"
  • To see the notes for this specification point, click here
• "The chromosomal basis of inheritance provides an understanding of the pattern of transmission of genes from parent to offspring"
  • To see the notes for this specification point, click here

• Genes can affect the phenotype of an organism
  • A gene codes for a single polypeptide
  • The polypeptide can affect the phenotype, eg. it could form part of an enzyme or a membrane transport protein
• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
• Genetic disorders are often caused by a mutation in a gene that results in a differently functioning or malfunctioning protein that alters the phenotype of the individual
• Most genetic diseases are caused by recessive alleles on autosomal chromosomes
  • This means that an individual would need two copies of the recessive allele in order to develop the disease
  • Individuals that are heterozygous do not suffer from the disease but are carriers and can pass the recessive allele on to the next generation
  • A disease determined by a recessive allele includes cystic fibrosis or Tay-Sachs
• Some diseases are caused by dominant
  • This means that only one copy of the allele is required in order to develop the disease and this one copy can also be passed on to the next generation
  • Individuals that are homozygous dominant, will suffer from the disease and will also pass the allele on to the next generation with 100% probability
  • A disease determined by a dominant allele includes Huntington's disease
• It is also possible, but rare, for a disease to be caused by codominant alleles
  • This means that in individuals with heterozygous genotype, both alleles are expressed in the phenotype
  • Therefore giving a 3rd phenotype that is different from the homozygous phenotypes
  • A disease determined by codominant alleles includes sickle cell anemia
• The genes which causes some genetic diseases are found on the sex chromosomes
  • This means they affect males and females differently
  • Examples of sex-linked diseases include hemophilia and color blindness

Cystic fibrosis

• Cystic fibrosis is a genetic disorder of cell membranes caused by a recessive allele of the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene located on chromosome 7
•  Cystic fibrosis sufferers have a lower life expectancy because of the strain the condition places on their breathing systems (and other systems)
  • This gene codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive enzymes
  • A mutation in the CFTR gene leads to production of non-functional chloride channels
  • The faulty allele is just 4 bases different from the fully functional allele
  • This shows the dramatic effect that even a small change in nucleotide sequence can have on an individual
• This reduces the movement of water by osmosis into the secretions
• The result is that the cells produce large amounts of thick, sticky mucus in the air passages, the digestive tract and the reproductive system
• Because cystic fibrosis is determined by a recessive allele, this means
  • People who are heterozygous won’t be affected by the disorder but are carriers
  • People must be homozygous recessive in order to display the disorder
  • If both parents are carriers the chance of them producing a child with cystic fibrosis is 1 in 4, or 25 %
  • If only one of the parents is a carrier with the other parent being homozygous dominant, there is zero chance of producing a child with cystic fibrosis, as the recessive allele will always be masked by the dominant allele

Cystic Fibrosis Inheritance Punnett Square

Cystic fibrosis is a genetic disorder caused by a recessive allele

Huntington's Disease

• Huntington’s disease is a genetic condition that develops as a person ages
• Usually a person with the disease will not show symptoms until they are 30 plus years old
• An individual with the condition experiences neurological degeneration; they lose their ability to walk, talk and think
• The disease is ultimately fatal
• It has been found that individuals with Huntington's disease have abnormal alleles of the HTT gene
  • The HTT gene codes for the protein huntingtin which is involved in neuronal development
• The abnormal allele is dominant over the normal allele
  • If an individual has one abnormal allele present they will suffer from the disease
  • If only one parent is a carrier of the dominant allele, there is still a 1 in 2 or 50% chance of producing a child with the disease

Huntington's Disease Inheritance Punnet Square

Huntington’s is caused by a dominant allele

Non-Disjunction

• Nondisjunction occurs when chromosomes fail to separate correctly during meiosis
• This can occur in either anaphase I or anaphase II, leading to gametes forming with an abnormal number of chromosomes
  • The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
  • These gametes will have a different number of chromosomes compared to the normal haploid number
• If the abnormal gametes are fertilized, then a chromosome abnormality occurs as the diploid cell (zygote) will have the incorrect number of chromosomes

Nondisjunction Compared to Normal Chromosome Separation Diagram

Chromosomes failing to separate properly during meiosis can result in gametes with the incorrect number of chromosomes

Down Syndrome

• A key example of a nondisjunction chromosome abnormality is Down syndrome, also called Trisomy 21
• Nondisjunction occurs during anaphase I (in this case) and the 21st pair of homologous chromosomes fail to separate
• Individuals with this syndrome have a total of 47 chromosomes in their cells as they have three copies of chromosome 21
• The impact of trisomy 21 can vary between individuals, but some common features of the syndrome are physical growth delays and reduced intellectual ability
  • Individuals can also suffer from issues with sight or hearing
• There are other examples of nondisjunction which result in trisomy
  • Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) are very serious syndromes which result in many physical disabilities and developmental difficulties
• The risk of chromosomal abnormalities increases significantly with age
  • The age of the mother is particularly important in the case of Down Syndrome as nondisjunction is more likely to happen in older ova
• Karyotyping of the chromosomes in fetal cells can be used to identify chromosomal abnormalities
  • Fetal cells may be obtained by performing an amniocentesis or by chorionic villus sampling

Down Syndrome Karyotype Diagram

The karyotype of an individual with Down Syndrome

Red-green color blindness

• The gene which is responsible for synthesizing the photoreceptor proteins of the eye, is found on the X chromosome
• The photoreceptor proteins are made in the cone cells of the eye and detect the specific wavelengths of light entering the eye
• Red-green color blindness is caused by a recessive allele of this gene
• Males are more likely to be red-green color blind as they only possess 1 allele for the gene, whereas females have 2 alleles and need to inherit 1 faulty allele from both parents in order to be color blind

Color-blindness Inheritance Punnet Square

Punnett grid showing the inheritance of colorblindness, an X-linked condition

Hemophilia

• Hemophilia is a well known sex-linked disease
• There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
• There are two alleles for factor VIII, the dominant F allele which codes for normal factor VIII and the recessive f allele which results in a lack of factor VIII
• When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can't clot normally
• If males have an abnormal allele they will have the condition as they have only one copy of the gene
• Females can be heterozygous for the faulty gene and not suffer from the condition but act as a carrier
• This means that hemophilia is a potentially fatal genetic disease which affects males more than females

Monohybrid Punnett Square with Sex-linkage Table

Hemophilia is carried on the X chromosome so males only carry one allele for this gene

Predicted ratio of phenotypes in offspring

1 female with normal blood clotting : 1 carrier female : 1 male with hemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring: 1 X^FX^F : 1 X^FX^f : 1 X^FY : 1 X^fY