BIO 301 — Genetics II: Lecture Note

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BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: BIO 301 Course title: Genetics II Course credit load: 2 Units Series 1: Human Genetics and Pedigree Analysis • Part 1.1: Aspects of Human Genetics o Sub Part 1.1.1: The human karyotype and chromosomal disorders o Sub Part 1.1.2: Autosomal inheritance patterns o Sub Part 1.1.3: Sex-linked inheritance • Part 1.2: Pedigree Analysis o Sub Part 1.2.1: Pedigree symbols and conventions o Sub Part 1.2.2: Identifying inheritance patterns from pedigrees o Sub Part 1.2.3: Worked pedigree analysis examples • Part 1.3: Chromosomal Abnormalities o Sub Part 1.3.1: Numerical chromosomal abnormalities o Sub Part 1.3.2: Structural chromosomal abnormalities o Sub Part 1.3.3: Sex chromosome abnormalities • Part 1.4: Genetic Counselling and Screening o Sub Part 1.4.1: Principles of genetic counselling o Sub Part 1.4.2: Prenatal diagnosis techniques o Sub Part 1.4.3: Genetic screening programmes

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction Human genetics applies the general principles of heredity to the human species, with the goal of understanding how traits and diseases are transmitted through families and populations. Unlike experimental organisms, humans cannot be bred to order; instead, genetic analysis in humans relies primarily on pedigree analysis, the systematic examination of family trees showing the presence or absence of traits across generations, combined with chromosomal and molecular investigation. This Series introduces the human karyotype, autosomal and sex-linked inheritance patterns, the conventions and interpretation of pedigrees, the major categories of chromosomal abnormality, and the principles and techniques of genetic counselling and screening. This foundation supports the gene interaction, molecular genetics, and biotechnology topics addressed in the remaining Series of this course. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 describe the human karyotype and distinguish autosomal from sex-linked inheritance patterns, • SLO 1.2 use standard pedigree conventions to analyse family trees and identify inheritance patterns, • SLO 1.3 describe numerical and structural chromosomal abnormalities and their phenotypic consequences, and • SLO 1.4 explain the principles of genetic counselling, prenatal diagnosis, and genetic screening.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.1 Aspects of Human Genetics 1.1.1 The human karyotype and chromosomal disorders The normal human karyotype consists of 46 chromosomes (2n = 46) arranged as 23 pairs: 22 pairs of autosomes (non-sex chromosomes, numbered 1 to 22 from largest to smallest based on size and banding pattern) and one pair of sex chromosomes (XX in females, XY in males); a karyotype is prepared by arresting dividing cells at metaphase, staining the chromosomes, photographing and then arranging the image so chromosome pairs can be systematically examined for number and structural integrity. The human genome contains approximately 3 billion base pairs and an estimated 20,000 to 25,000 protein-coding genes, distributed unevenly across the 23 chromosome pairs; the sex chromosomes differ markedly in gene content, with the X chromosome carrying approximately 800 to 900 genes (many unrelated to sex determination) while the Y chromosome is much smaller and carries far fewer genes, mostly related to male sex determination and fertility. Fill in the blank: The normal human karyotype consists of ________ chromosomes arranged as 22 pairs of autosomes and one pair of sex chromosomes. 1.1.2 Autosomal inheritance patterns Autosomal dominant inheritance is characterised by: the trait appearing in every generation (no skipping), affected individuals usually having at least one affected parent, approximately half the offspring of an affected parent being affected (on average), and equal frequency in males and females; examples include Huntington disease (caused by a dominant allele on chromosome 4 producing an abnormal huntingtin protein), familial hypercholesterolaemia, and polydactyly (extra digits). Autosomal recessive inheritance is characterised by: the trait often skipping generations (carriers are phenotypically normal), both parents of an affected individual typically being unaffected carriers, approximately one quarter of the offspring of two carrier parents being affected, and equal frequency in males and females; examples include cystic fibrosis (defective CFTR chloride

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS channel protein), sickle cell disease (abnormal haemoglobin), phenylketonuria (PKU, deficient phenylalanine hydroxylase), and albinism (deficient melanin production). Fill in the blank: Autosomal ________ inheritance often skips generations since carriers are phenotypically normal, with approximately one quarter of offspring from two carriers being affected. 1.1.3 Sex-linked inheritance X-linked recessive inheritance is the most common form of sex-linkage in human genetics: because males have only one X chromosome, a single recessive allele on the X is sufficient to produce the affected phenotype (hemizygous males), while females require two copies of the recessive allele to be affected; carrier females (heterozygous, one normal and one recessive allele) are typically unaffected but can transmit the allele; affected males cannot transmit the trait to their sons (who receive the Y from their father) but all their daughters will be carriers. Examples of X-linked recessive conditions include haemophilia A and B (clotting factor deficiencies), red-green colour blindness (defects in cone photoreceptor pigment genes), and Duchenne muscular dystrophy (deficiency of the dystrophin protein); X-linked dominant conditions (rarer) affect both males and females but are often more severe in males, while Y-linked (holandric) traits pass exclusively from father to all sons and are extremely rare, as the Y chromosome carries very few genes. Fill in the blank: In X-linked recessive inheritance, ________ males are affected by a single recessive allele because they have only one X chromosome, while carrier females are typically unaffected.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.1: Human karyotype and inheritance patterns Pilot questions 1.1 1. State the number of chromosomes in the normal human karyotype and how they are arranged. 2. Name three characteristics of autosomal dominant inheritance. 3. Name three characteristics of autosomal recessive inheritance. 4. Explain why X-linked recessive conditions are more common in males than females. 5. Give two examples each of autosomal dominant, autosomal recessive, and X-linked recessive conditions. 1.2 Pedigree Analysis 1.2.1 Pedigree symbols and conventions Standard pedigree symbols allow family inheritance patterns to be represented diagrammatically across multiple generations; the conventions are: squares represent males, circles represent

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS females, a diamond or unspecified shape is used when sex is unknown; filled (shaded) symbols represent affected individuals, unfilled symbols represent unaffected individuals, and half-filled symbols may indicate carriers in some conventions; horizontal lines connect mating pairs, vertical lines connect parents to offspring, and a horizontal sibship line connects siblings. Additional conventions include: a double horizontal line between mating pairs indicating a consanguineous union (between related individuals), diagonal lines through a symbol indicating a deceased individual, a dot within a circle or small filled dot indicating an obligate or confirmed carrier female, numbers within symbols indicating multiple individuals of the same type in the same generation, and Roman numerals labelling successive generations (I for the oldest generation shown, II for the next, and so on) with Arabic numerals identifying individuals within each generation. Fill in the blank: In pedigrees, ________ represent males and circles represent females, with filled symbols indicating affected individuals. 1.2.2 Identifying inheritance patterns from pedigrees Systematic pedigree interpretation follows a logical sequence of pattern recognition: if the trait appears in every generation without skipping, autosomal dominant is most likely; if the trait skips generations with unaffected carrier parents, autosomal recessive is most likely; if the trait is more common in males and transmitted through unaffected females, X-linked recessive is most likely; if the trait appears in females and males but more severely in males, X-linked dominant should be considered; if the trait passes exclusively from fathers to all sons, Y-linkage is suggested. Key clues in pedigrees include: the sex ratio of affected individuals (equal for autosomal, male-biased for X-linked recessive), the presence of affected individuals with two unaffected parents (obligate carriers, diagnostic of recessive inheritance), consanguineous matings (increasing the probability of homozygosity for rare recessive alleles), and the proportion of affected offspring from different cross types, compared against expected Mendelian ratios.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Fill in the blank: If a trait appears in every generation without skipping, ________ dominant inheritance is most likely; if it skips generations with unaffected carrier parents, recessive is most likely. 1.2.3 Worked pedigree analysis examples A three-generation pedigree showing a trait in Generation I father, Generation II son and daughter, and Generation III grandson (son of the II son) with no unaffected carrier parents at any point is consistent with autosomal dominant inheritance (trait in every generation, both sexes affected, approximately half offspring affected); a pedigree showing two unaffected parents (Generation II) with affected children of both sexes is consistent with autosomal recessive inheritance, making the parents obligate carriers. A pedigree showing unaffected females in Generation I and II whose brothers and sons are affected, while daughters and granddaughters who are unaffected may

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