Chromosomal Inheritance (College Board AP® Biology): Study Guide

Genetic variation

• Genetic variation is crucial for evolution and adaptation in populations

• Within populations genetic variation can result from different genetic processes, including

  • segregation of chromosomes during meiosis

  • independent assortment of chromosomes

  • fertilization

• These topics are also covered in the topic on Meiosis and genetic diversity

Segregation

• Segregation occurs in Anaphase I of meiosis when homologous chromosomes are pulled apart

• The process is random and this ensures that offspring inherit a mixture of both maternal and paternal alleles

Independent assortment

• Independent assortment is the random orientation of homologous chromosome pairs along the equator of the cell during metaphase I of meiosis

  • The alignment of each pair is independent of other pairs

  • Therefore each gamete will have a different combination of chromosomes, depending on how they have aligned

• Humans have 23 pairs of chromosomes, this results in 2²³ (8.4 million) possible combinations of maternal and paternal chromosomes which leads to high genetic diversity

Fertilization

• Fertilization involves the random fusing of two gametes

• Each gamete will have a unique combination of alleles due to segregation and independent assortment

• The resulting zygote will inherit a completely new combination of alleles from both parents, which further leads to genetic variation

Genetic disorders

• Genes affect the phenotype of an organism

  • A gene codes for a single polypeptide chain

  • The polypeptide can affect the phenotype, e.g. it could form part of an enzyme or a membrane transport protein

  • Gene mutations lead to a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide and therefore a phenotype

• 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

• Some diseases are caused by dominant alleles

• The genes which causes some genetic diseases are found on the sex chromosomes

  • This means they affect males and females differently

  • E.g. hemophilia and color blindness

Examples of genetic disorders

Sickle cell anemia

• Sickle cell anaemia is a condition that causes individuals to have frequent infections, episodes of pain and anaemia

• Humans with sickle cell anaemia have abnormal haemoglobin in their red blood cells

• β-globin is a polypeptide found in haemoglobin that is coded for by the gene HBB

• There is an abnormal allele for the gene HBB which produces a slightly different amino acid sequence to the normal allele

  • This change in amino acid sequence results in an abnormal β-globin polypeptide

  • The abnormal β-globin in haemoglobin affects the structure and shape of the red blood cells

    • They are pulled into a half moon shape

    • They are unable to transport oxygen around the body

    • They stick to each other and clump together blocking capillaries

• A homozygous individual that has two abnormal alleles for the HBB gene produces only sickle cell haemoglobin

  • They have sickle cell anaemia and suffer from the associated symptoms

• A heterozygous individual that has one normal allele and one abnormal allele for the HBB gene will produce some normal haemoglobin and some sickle cell haemoglobin

  • They are a carrier of the allele

  • They may have no symptoms

Tay-Sachs disease

• Tay-Sachs disease is a genetic disorder caused by a mutation in the HEXA gene, resulting in a deficiency of the enzyme hexosaminidase A

• The disease is an autosomal recessive disorder, meaning both parents must carry a mutated copy of the HEXA gene to pass it on to their child

• Symptoms of Tay-Sachs typically appear by 3 to 6 months of age including

  • loss of motor skills (like reaching or crawling)

  • exaggerated startle response

  • muscle weakness and stiffness

  • vision and hearing loss

  • seizures

• There is no cure for Tay-Sachs and treatments focus on managing symptoms and providing supportive care

Huntington's Disease

• Huntington’s disease is a genetic condition that develops as a person ages

• 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

X-linked color blindness

• X-linked color blindness is a sex-linked genetic disorder

• 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 one allele for the gene

• Females have two alleles and would need to inherit one faulty allele from both parents in order to be color blind

Trisomy 21/Down syndrome

• Down syndrome, also called Trisomy 21 is an example of a nondisjunction chromosome abnormality

  • 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

• In Down's syndrome nondisjunction occurs during anaphase I 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

• 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