11.4 Inheritance in Humans
Human cells contain 46 chromosomes, comprising 44 autosomes and 2 sex chromosomes. The arrangement and display of these chromosomes is called a karyotype. Variations or abnormalities can occur when chromosomes fail to separate correctly during meiosis, leading to genetic disorders such as Down syndrome, Turner syndrome, and Klinefelter syndrome. In addition, human inheritance patterns govern traits such as ABO blood groups, Rhesus factor, thalassemia, tongue rolling, earlobe types, and sex-linked traits like colour blindness and haemophilia, which can be tracked across generations using family pedigrees.
Learning Objectives
- Identify and differentiate between autosomes and sex chromosomes in the human karyotype.
- Explain the causes and chromosomal characteristics of genetic disorders resulting from nondisjunction (Down syndrome, Turner syndrome, and Klinefelter syndrome).
- Analyse the inheritance of ABO blood groups, Rhesus factor, and thalassemia using genetic diagrams and genotypic ratios.
- Explain sex determination and sex-linked inheritance of traits located on the X chromosome, specifically red-green colour blindness and haemophilia.
- Describe Mendelian inheritance in tongue rolling and earlobe attachment, and interpret family pedigree charts to track inherited traits across generations.
Summary of Human Genetic Disorders Caused by Nondisjunction
Compare the chromosome counts, sex chromosome compositions, and gender/phenotype differences among Down syndrome, Turner syndrome, and Klinefelter syndrome.
Down syndrome
Trisomy 21: an extra chromosome at pair 21. Total chromosomes: 47.
Turner syndrome
Monosomy X: one X chromosome is missing. Total chromosomes: 45.
Klinefelter syndrome
Extra X chromosome in the sex chromosome pair. Total chromosomes: 47.
Short Explanation
Human chromosomes and karyotype
Human somatic cells contain 46 chromosomes: 44 autosomes and 2 sex chromosomes. A karyotype is the number and structure of chromosomes present in a cell nucleus, arranged in pairs based on homologous chromosomes in terms of size, centromere location, and banding pattern.
Autosomes
Chromosome pairs 1 to 22. They control somatic characteristics such as blood groups, height, and skin colour. They vary in size and length.
Sex chromosomes
Chromosome pair 23. Females have two identical X chromosomes, \(44 + \text{XX}\). Males have one X and one smaller Y chromosome, \(44 + \text{XY}\). The Y chromosome carries only male sex-determining genes.
Nondisjunction
The failure of homologous chromosomes to separate during anaphase I, or sister chromatids to separate during anaphase II. Gametes produced have an abnormal number of chromosomes, \(22\) or \(24\). Fertilisation with a normal gamete, \(n = 23\), produces zygotes with \(45\) or \(47\) chromosomes.
Sex determination
Male gametes are haploid: \(22 + \text{X}\) or \(22 + \text{Y}\). Female gametes are haploid: all contain \(22 + \text{X}\). Gender is determined during fertilisation. The ratio of female to male offspring is \(1:1\).
ABO blood groups
Controlled by a single gene with multiple alleles: \(I^A\), \(I^B\), and \(I^O\). An individual possesses only two alleles. \(I^A\) and \(I^B\) are dominant over \(I^O\); \(I^O\) is recessive. \(I^A\) and \(I^B\) are codominant to each other.
Rhesus factor (Rh)
Involves antigen D on the surface of red blood cells. Controlled by a pair of alleles following Mendelian principles: \(\text{Rh}^+\) is dominant and \(\text{Rh}^-\) is recessive. Rhesus positive: \(\text{Rh}^+\text{Rh}^+\) or \(\text{Rh}^+\text{Rh}^-\). Rhesus negative: \(\text{Rh}^-\text{Rh}^-\).
Thalassemia
An inherited autosomal disease caused by a gene mutation on chromosome 11 or 16, resulting in abnormal and low haemoglobin levels. Thalassemia minor is heterozygous and asymptomatic. Thalassemia major is homozygous recessive and shows severe symptoms.
Sex-linked inheritance
Traits controlled by genes located on the non-homologous region of the X chromosome. Because the Y chromosome is shorter and lacks homologous alleles for these genes, any trait carried by a single allele on the X chromosome in a male is directly expressed.
Family pedigree
A lineage flowchart over multiple generations illustrating ancestral relationships and pattern of trait transmission. Square = male, circle = female; shaded = affected individual; unshaded = normal individual; half-shaded = carrier female.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the chromosomal composition of an individual with Turner syndrome, and what is the gender of the individual?
A man with blood group A (heterozygous) marries a woman with blood group B (heterozygous). What are the possible blood groups of their children, and what is the phenotypic ratio?
Why are males more frequently affected by red-green colour blindness and haemophilia compared to females?
Common Mistakes
- Writing the karyotype of Turner syndrome as \(45 + \text{XO}\) instead of \(44 + \text{XO}\) (total 45 chromosomes: 44 autosomes + 1 X chromosome).
- Forgetting to write sex chromosomes when representing genotypes of sex-linked traits, for example writing \(B\) or \(b\) instead of \(X^B\) and \(X^b\).
- Confusing codominance in ABO blood group with incomplete dominance; \(I^A\) and \(I^B\) are codominant because both antigens are fully and simultaneously expressed on red blood cells.
- Stating that males can be carriers for sex-linked recessive conditions like haemophilia; males cannot be carriers because they have only one X chromosome—they are either normal, \(X^H Y\), or affected, \(X^h Y\).
Concept Misunderstandings
Down syndrome only affects males because some karyotype charts show XY.
Down syndrome is an autosomal disorder caused by trisomy 21, so it affects both males, \(45 + \text{XY}\), and females, \(45 + \text{XX}\), equally.
The father and mother contribute unequally to determining whether a child gets an autosome-linked disease.
Autosomal traits are inherited equally from maternal and paternal autosomes; however, for sex determination, only the father's sperm, carrying X or Y, determines the genetic sex of the offspring.
Thalassemia is a sex-linked disease.
Thalassemia is an autosomal recessive disorder caused by mutations on autosomes, chromosome 11 or 16, not the sex chromosomes.
Summary
- Somatic cells carry 46 chromosomes: 44 autosomes and 2 sex chromosomes.
- Nondisjunction causes abnormal chromosome numbers: Down syndrome, 47 chromosomes, trisomy 21; Turner syndrome, 45 chromosomes, \(44 + \text{XO}\); Klinefelter syndrome, 47 chromosomes, \(44 + \text{XXY}\).
- ABO blood grouping is controlled by multiple alleles, \(I^A, I^B, I^O\), displaying codominance between \(I^A\) and \(I^B\).
- Rhesus factor follows Mendelian inheritance with \(\text{Rh}^+\) dominant over \(\text{Rh}^-\).
- Thalassemia is an autosomal recessive mutation on chromosome 11 or 16 affecting haemoglobin synthesis.
- Sex is determined at fertilisation by whether an X-bearing or Y-bearing sperm fertilises the ovum, giving a \(1:1\) ratio.
- Sex-linked genes on the non-homologous region of the X chromosome, such as colour blindness and haemophilia, are more frequently expressed in males.
- Family pedigrees map inheritance of phenotypes and genotypes across generations to trace dominant and recessive traits.
Short Activity
Objective: test your understanding of human chromosomal abnormalities, sex-linked inheritance, and genetic descriptions.
A. Objective Quiz
Which of the following chromosome compositions represents an individual with Klinefelter syndrome?
A carrier female for haemophilia, \(X^H X^h\), marries a normal male, \(X^H Y\). What is the probability that their son will be haemophilic?
B. Fill in the Blanks
Down syndrome is also known as trisomy 21 because there is an extra chromosome present at chromosome pair number .
Thalassemia is an inherited disorder caused by a gene mutation located on autosome number 11 or .
C. Matching / Drag and Drop
Match each human genetic condition or characteristic to its correct chromosomal or genetic description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Genetic Condition or Trait to Its Description
Write the correct condition or trait next to each description.
| Chromosomal / Genetic Description | Genetic Condition / Trait |
|---|---|
| 1. Chromosomal abnormality with genotype \(44 + \text{XO}\) resulting in a female with 45 chromosomes | |
| 2. Sex-linked disorder caused by a recessive allele on the X chromosome leading to impaired blood clotting | |
| 3. Controlled by antigen D on red blood cells following Mendelian inheritance with \(\text{Rh}^+\) dominant to \(\text{Rh}^-\) | |
| 4. Inherited via multiple alleles, \(I^A, I^B, I^O\), exhibiting codominance between allele \(I^A\) and allele \(I^B\) |
Answer choices: Turner syndrome • Haemophilia • Rhesus factor • ABO blood group
