ICSE Biology Lecture – Genetics (One-Shot)

Flashcards cover key ICSE genetics concepts: basic terms, Mendel’s work, mono- & dihybrid crosses, three Mendelian laws, chromosomes, sex determination, pedigree symbols, X-linked disorders (colour blindness & haemophilia), mutation examples, and important definitions.

Who is known as the “Father of Genetics”?

Gregor Johann Mendel

What branch of biology studies the transmission of inherited traits from parents to offspring?

Genetics – the study of heredity

Define heredity in genetic terms.

The transfer of genetic characters (genes/chromosomes) from parents to their offspring

What is a gene?

A specific DNA segment that codes for a protein, enzyme or hormone and governs a particular trait

How many genes (approx.) are present in humans?

≈ 19,000 genes

What is a genome?

The complete set of DNA (all genes + non-coding regions) in an organism

What is a karyotype?

The organised visual display of the complete set of chromosomes of a cell

How many chromosomes are present in human body cells?

46 chromosomes (23 pairs)

How many chromosome pairs in a garden pea (Pisum sativum)?

7 pairs (14 chromosomes)

Differentiate autosomes and sex chromosomes.

Autosomes: 22 homologous pairs in humans that control somatic traits; Sex chromosomes: 1 pair (XX or XY) that determine sex

What genetic combination produces a human male?

XY (one X and one Y chromosome)

What genetic combination produces a human female?

XX (two X chromosomes)

What is the genotype of a heterozygous tall pea plant if ‘T’ = tall and ‘t’ = dwarf?

Tt

Define allele.

Alternative forms of the same gene located at the same locus on homologous chromosomes

What is meant by dominant allele?

An allele that expresses its effect even in the presence of its alternative recessive allele

What is a recessive allele?

An allele whose effect is masked in the presence of a dominant allele and only expresses when homozygous

State Mendel’s Law of Segregation.

Allele pairs separate (segregate) during gamete formation so each gamete carries only one allele for each gene

State Mendel’s Law of Independent Assortment.

Alleles of different genes assort independently of one another during gamete formation when on separate chromosomes

State Mendel’s Law of Dominance.

In a heterozygote, one allele (dominant) masks the expression of the other (recessive)

In a monohybrid F₂, what is the phenotypic ratio?

3 : 1 (dominant : recessive)

In a monohybrid F₂, what is the genotypic ratio?

1 : 2 : 1 (homozygous dominant : heterozygous : homozygous recessive)

In a dihybrid F₂, what classic phenotypic ratio did Mendel obtain?

9 : 3 : 3 : 1

Name any two of Mendel’s seven pea characters.

Examples: Seed shape (round/wrinkled), seed colour (yellow/green), pod shape (inflated/constricted), pod colour (green/yellow), flower colour (purple/white), flower position (axial/terminal), plant height (tall/dwarf)

Define phenotype.

The observable physical expression of a genotype (e.g., tall, round-yellow)

Define genotype.

The genetic makeup (allele combination) of an organism for a trait (e.g., TtYY)

What term describes chromosomes of the same size, shape and gene loci—one from each parent?

Homologous chromosomes

What are gametes and how many chromosomes do they contain in humans?

Sex cells (sperm/ovum) produced by meiosis; each carries 23 chromosomes (haploid)

Which symbol is used in pedigree charts for males?

A square

What does a filled (shaded) symbol in a pedigree represent?

The individual expresses (is affected by) a particular trait/disorder

What is colour blindness and how is it inherited?

Inability to distinguish certain colours (usually red–green); an X-linked recessive disorder

Why are males more often colour-blind than females?

Males have only one X chromosome; a single recessive allele on X causes the disorder, whereas females require two affected alleles

What clotting disorder is called the “Royal disease”?

Haemophilia – an X-linked recessive disorder

Which clotting factors are missing/defective in haemophilia?

Clotting factor VIII or IX

Define mutation.

A sudden heritable change in DNA sequence, gene, or chromosome structure/number

What base change causes sickle-cell anaemia?

A point mutation where the codon GAG (glutamic acid) changes to GTG (valine) in the β-globin gene

How does sickle-cell anaemia affect red blood cells?

RBCs become sickle-shaped, break easily and block capillaries, leading to anaemia and pain

What is polydactyly?

A hereditary condition (often due to mutation) in which an individual has extra fingers or toes

Give the chromosome complement of a carrier female for colour blindness.

XᶜX (one normal X, one X with colour-blind allele)

What is the role of meiosis in genetics?

It halves chromosome number to form gametes and creates genetic variation by segregation and crossing-over

Why did Mendel choose pea plants for his experiments? (Give any two reasons)

Many contrasting traits; easy to grow; short life cycle; self-pollinating but can be cross-pollinated; large number of seeds

Which genotype represents a homozygous recessive dwarf pea plant?

tt

What term describes a person who carries a recessive disease allele but is phenotypically normal?

Carrier (heterozygous for the disorder)