Genetics and Inheritance

• This lecture covers genetics and inheritance in the context of molecular and cellular genetics, within the Foundations of Medical Science.

Teaching Objectives

• To understand the chromosomal basis of inheritance and the gene as the unit of inheritance.
• State the human karyotype and the consequences of abnormal karyotypes.
• Understand the importance of gene variants, also known as alleles.
• Recognize recessive and dominant modes of inheritance in simple monogenic diseases.

Inherited Disease

• Genotype refers to the complete set of genes within an organism's genome.
• Phenotype is the observable functional result of gene expression, either from a single gene or a combination of genes.
• Genetic diseases arise from inherited gene variants, also known as mutations, which lead to abnormal gene function.
• Monogenic disorders stem from a problem on a single gene, cystic fibrosis is an example.
• Polygenic disorders arise from the combination of many different genes, Type II diabetes is an example.
• Genotype can influence resistance to disease, like the absence of the CCR5 chemokine receptor which can provide immunity to HIV infection.
• Genotype can affect how someone responds to drugs, giving rise to the field of pharmacogenetics.

Pharmacogenetics

• Cytochrome P450 enzymes are commonly involved in the metabolism and detoxification of drugs within the body.
• CYP450 enzymes are of great importance.
• There are numerous variants of the CYP450 gene which translate into varied enzyme activity.
• Patients who have specific CYP450 variants may show a decreased efficiency in drug metabolism.
• For instance, a normal dose of the anticoagulant drug warfarin can lead to excessive bleeding in individuals with certain CYP450 variants.
• Genotyping can be carried out to identify which variants are present, to inform the process of therapy.
• Personalized medicine involves prescribing drugs based on an individual's genotype.

Packaging of DNA

• DNA is packaged within the nucleus alongside positively charged histone proteins.
• DNA plus histones results in chromatin.
• An octamer of histone proteins, specifically two of each H2A, H2B, H3, and H4, forms a core around which DNA is wound.
• Histone octamer is referred to as nucleosome.
• H1 histone proteins function as links between nucleosomes.
• Nucleosomes are compacted to form chromatin fibers.
• Chromatin fiber loops out from matrix protein, which results in individual chromosomes.

Human Karyotype

• A human karyotype consists of 46 chromosomes which includes 23 from each parent.
• Human cells are diploid (2n).
• There are 22 pairs of autosomes.
• There is 1 pair of sex chromosomes.
• Staining chromosomes at metaphase with Giemsa dye (G-banding) results in alternating light and dark bands which are characteristic for each chromosome.
• Cytogenetics is the study of a cell's genetic components through visualization and analysis of chromosomes.
• A normal female karyotype is 46 XX.
• A normal male karyotype is 46 XY.
• Chromosome 1 is the largest, while chromosome 22 is the smallest.

Organisation of Chromosomes

• After DNA replication, each chromosome consists of two identical chromatids.
• Two identical chromatids are linked together at the centromere.
• Chromosome arms have different lengths these are referred to as short (p) and long (q).
• A gene can be located by citing the G-banding pattern and the arm of the chromosome, such as the alpha globin genes at 16p13.3.

Mitosis

• Mitosis leads to making 2 genetically identical cells, each containing 46 chromosomes (diploid : 2n).
• The process involves one round of DNA replication and chromosomal segregation
• DNA replicates and Chromosomes duplicate to create chromatids in prophase
• Chromosomes align at the spindle's center during metaphase.
• Chromatids divide and separate toward opposite poles of the cell during anaphase.
• The nuclear membrane reforms and the cell divides during telophase.

Meiosis

• Meiosis results in making 4 cells, each containing 23 chromosomes (haploid : n) that are genetically variable.
• The process involves one round of DNA replication and two rounds of chromosomal segregation.
• Homologous chromosome pairs line up and exchange DNA during prophase I, a process called crossing over.
• Homologous chromosome pairs align at the center of the spindle during metaphase I.
• Chromosomes separate and move toward opposite poles of the cell during anaphase I.
• Sister chromatids remain together, followed by nuclear membrane reforming and cell splitting.
• Chromosomes line up at the spindle's center during metaphase II.
• Chromatids separate and move toward opposite poles of the cell.
• The nuclear membrane reforms at the end of telophase II, eventually producing 4 cells.

X-chromosome Dosage Compensation

• Genes from maternal and paternal chromosomes are usually expressed.
• X-chromosome dosage compensation is a mechanism to prevent female cells from having twice the amount of X-linked gene expression.
• Female cells are 46 XX in their karyotype, however only one X chromosome is active.
• Given only one X chromosome is active in both males and females, the level of X-linked gene expression is roughly the same.
• Early in embryological development, one of the two X chromosomes is randomly deactivated.
• The Xist ncRNA gene makes RNA which triggers transcriptional silencing of one of the X chromosomes.
• The inactivation pattern is maintained by cells descended from the early development stages.
• Females are mosaics for X-linked genes, with roughly 50% of cells having a maternally inherited X-active and 50% having a paternally inherited X-active.

Biological Sex Determination

• The presence of a Y chromosome determines male sex.
• Extremely rare mutations can identify the region of the Y chromosome that determines sex.
• Males having a 46 XX karyotype have a part of the Y chromosome translocated to the X chromosome.
• Females having a 46 XY karyotype have a piece of the Y chromosome deleted.
• The SRY gene is located on the Y chromosome and encodes a protein that acts as a testis-determining factor (TDF).
• SRY is the only Y region needed for male development.

Abnormal Karyotypes

• A complete set of chromosomes is euploidy, while an irregular number is aneuploidy.
• Aneuploidies are caused by non-disjunction where homologous chromosomes fail to separate properly during meiosis.
• Aneuploidies account for account for 50% of spontaneous abortions.
• Trisomies where there are 3 copies of a chromosome, can lead to live births.
• Trisomy 13 (Patau syndrome) is signified by either 47 XX +13 or 47 XY +13, signs are heart defects, neurological issues and a mean survival of approximately 130 days.
• Trisomy 18 (Edward’s syndrome) which is either 47 XX +18 or 47 XY +18, the signs are frequent death in utero, heart and intestinal abnormalities, and approximately 10% of live births survive for 12 months
• Trisomy 21 (Down syndrome) which is either 47 XX +21 or 47 XY +21, produces variable phenotype, cardiac abnormalities and an increased risk for acute leukaemia.
• Chromosomes 13, 18 and 21 are small and therefore the number of genes in triplicate is small, which is why they are the only viable trisomies.

Karyotype Determination

• Amniocentesis is carried out during weeks 15-20 of pregnancy to check for chromosomal abnormalities.
• Removal of amniotic fluid, isolation of fetal cells and quantification of chromosomes are carried out in amniocentesis.
• The risk of Down syndrome increases from 1:600 at maternal age 33 to 1:50 at age 43.
• 95% of aneuploid conceptions can be attributed to non-disjunction during oogenesis.
• Spermatogenesis continues throughout life after puberty.
• Oogenesis is largely complete at birth, while oocytes are suspended after the first cell division.
• Second division is only completed after an egg is fertilized.

Monogenic Disorders – Pedigree Symbols

• Monogenic disorders are linked to a variation or mutation in a single gene.
• The modes of inheritance are autosomal recessive, autosomal dominant, and X-linked recessive.

Autosomal Recessive

• Autosomal recessive traits are often rare in a pedigree.
• Autosomal recessive traits may skip generations.
• Autosomal recessive traits affect males and females.
• Autosomal recessive trait can be transmitted by either sex.
• Parents of affected individuals do not have the disease, because they are carriers.

Cystic Fibrosis (CF)

• Cystic fibrosis (CF) stems from a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.
• The locus of the CFTR gene is located on chromosome 7.
• Individuals inherit one allele from the father and the other from the mother.
• Sequence variations of a gene are referred to as alleles
• Genome contains extensive gene polymorphism which some alleles are non-functional, they are called mutant alleles or mutants.
• The normal functional allele can be called the wildtype allele.
• There are more than 500 mutant alleles of CFTR which the most common is CFTRΔ508, deletion of codon 508.
• Individuals must have two mutant alleles for CF to develop, which makes it an autosomal recessive disease.

CF Genetic Explanation

• Consider alleles of CTFR gene which are C representing wildtype and c representing the CFTRΔ508 mutation.
• The genotypes are either homozygous which is CC or cc, or heterozygous which is Cc.
• The CC genotype yields a normal phenotype.
• The Cc genotype also yields a normal phenotype
• The cc genotype yields an abnormal phenotype.
• The progeny ratio where both gene carriers is 3 normal and 1 having cystic fibrosis.
• Carrier rates of mutant alleles are very high approximately 1:25 for CFTR in northern Europe.
• Having a CFTR allele confers a selective evolutionary advantage.

CF Biological Explanation of Recessive Inheritance

• CFTR pumps Cl- out of cells, which leads to water exiting the cell through osmosis.
• Normally 50% of the CFTR is produced from the paternal allele and 50% is produced from the maternal allele.
• Cells that have a single mutant allele makes 50% of the normal amount of CFTR.
• 50% of CFTR function is enough to supply needed function.
• It is possible that an upregulation of the normal allele may compensate the lacking CFTR proteins.

Autosomal Dominant

• Autosomal dominant traits are considered frequent in pedigrees.
• Autosomal dominant affects individuals in each generation.
• Autosomal dominant affects males and females equally.
• Autosomal dominant trait can be transmitted by either sex.
• Parents of individuals that have a phenotype can thus pass it down to their children.

Huntington’s Disease (HD)

• Huntington’s Disease (HD) is a neurodegenerative disease which causes a gradual cognitive decline.
• HD is caused by mutations at the HD locus.
• A normal HD gene has about 28 repeats of CAG sequences.
• A mutant HD gene has more than 36 repeats of CAG sequences.
• The expansion of the polyglutamine region produces a version of huntingtin protein that aggregates and becomes neurotoxic.
• The inheritance of HD is an autosomal dominant trait.

HD Genetic Explanation

• The alleles of HD gene are H representing the wildtype and h which represent the 36+ CAG mutant.
• Homozygous genotypes are HH or hh, while heterozygotes are Hh.
• HH genotype gives a normal phenotype.
• Hh genotype gives an abnormal phenotype.
• hh genotype gives an abnormal phenotype
• The progeny ratio from an unaffected individual and a heterozygote affected individual is 2 normal and 2 Huntington's disease

HD Biological Explanation of Dominant Inheritance

• A patient that is a heterozygote with a single mutant allele (Hh) develops the disease.
• A mutant huntingtin protein with a pathological expanded polyglutamine region causes also causes the normal protein to aggregate and become neurotoxic.
• The mutant huntingtin protein aggregates with normal and mutant forms, causing a dominant and negative impact.
• Alternative causes for dominant recessive inheritance include haploinsufficiency, which the reduction to 50% of the normal protein is not sufficient for function.
• Somatic second hit is another cause, which is usually seen in cancer predisposition syndromes where patients will inherit a single mutant allele and a second mutation will somatically happen leading to cancer.

X-linked Recessive Inheritance

• In X-linked recessive inheritance, the trait is often rare in the pedigree.
• The trait tends to skip a generation.
• It is most frequently seen in males.
• Fathers are unable to pass it to their sons.
• Parents of affected individuals do not usually have the trait.

Haemophilia A

• Hemophilia A is a blot clotting disorder that is caused by a mutation in the factor VIII gene located on the X chromosome.
• Hemophilia A has X-linked recessive inheritance.
• Female carriers of the hemophilia A mutation contain half of the normal FXIII activity and therefore maintain normal blood clotting function.

Haemophilia A Genetic Explanation

• The alleles are H which is the wildtype and h which is the mutant.
• Female genotypes are HH, Hh, and hh, but male genotypes are HY and hY.
• The HH, Hh, and HY genotypes show have normal phenotypes.
• The hY and hh genotypes produce abnormal phenotypes.
• The result of progeny from a non carrier mother and affected father, shows that all males are unaffected and all females are carriers.
• The typical result of carrier mother and unaffected father yields 50% male offspring being affected and 50% female offspring are carriers.

Complex Disorders

• Most traits are polygenic and are dictated by multiple genes such as height.
• Multiple diseases shows states are multifactorial as they are both polygenic and influenced by other non-genetic conditions like obesity, diabetes as well as cardiovascular disease.
• Twin studies are useful to assess genetic factors versus environmental factors.
• In broad terms, environmental factors and genetics are identical between twins.
• Monozygotic twins have MZ are genetically identical.
• Dizygotic twins have DZ and are genetically non identical.
• Analysis on specific diseases within twin studies can allow greater insights to understand their link to genetics.
• Diseases that are more prevalent within MZ twin and DZ must be influenced by genetics.
• Inversely, diseases that show similar rates on both groups are controlled by environmental factors.

Concordance Rates in MZ and DZ

• Rheumatoid arthritis: MZ 32% and DZ 6%
• Schizophrenia: MZ 69% and DZ 10%
• Multiple sclerosis: MZ 28% and DZ 5%
• Diabetes (type II): MZ 70% and DZ 30%
• Cancer (all sites): MZ 12% and DZ 15%
• There is an apparent genetic component for all of the conditions above except for sporadic cancer, with MZ twins displaying a larger percentage and a stronger genetic component.

Summary

• The summary of the lesson is the nature of karyotypes.
• The summary of the lesson is also Mendelian genetic inheritance.