Genotypes and Inheritance Patterns Revision Notes for HSC SSCE Biology

Genotypes and Inheritance Patterns

Introduction to Mendelian inheritance

Inheritance patterns have fascinated scientists for over 170 years. Gregor Mendel, often called the father of genetics, proposed a mathematical model to predict the ratios of offspring from any two specific parents. His work laid the foundation for all modern inheritance patterns.

infoNote

Mendel's laws became particularly important when combined with Darwin's theory of evolution by natural selection. This combination led to the development of modern synthesis, which merged Mendelian genetics with evolutionary theory to create our current understanding of evolution (also called neo-Darwinism).

Modern genetics terminology

Before exploring inheritance patterns, it's essential to understand key genetic terms that form the foundation of modern genetics.

Genes and alleles

A gene is a segment of DNA on a chromosome that determines a particular characteristic. For example, in pea plants, one gene determines stem length, whilst another determines seed colour.

Alleles are different versions of the same gene. They:

Occur in pairs in diploid individuals
Occupy identical positions (called loci, singular: locus) on pairs of homologous chromosomes
Separate during gamete formation through meiosis
Appear individually in each haploid gamete
Pair up again during fertilisation when the diploid condition is restored

Genotype and phenotype

The genotype is the combination of alleles present in an organism's cells. In simple terms, it's the genetic makeup.

The phenotype is the physical appearance and observable characteristics resulting from gene expression. At a molecular level, phenotype represents the sum of all gene products (proteins and RNA) that are produced. These determine not only physical appearance but also behaviour and functioning.

chatImportant

Phenotype results from both genotype and environmental factors. For instance, a person's final height depends on their genetic potential and their nutrition during growth. Poor nutrition may prevent someone from reaching their genetically-determined height.

Homozygous and heterozygous genotypes

Mendel originally used the terms "pure-breeding" and "hybrid." In modern genetics, these are known as:

Homozygous: having two identical alleles (e.g., $TT$ or $tt$ ). The term comes from homo (the same) and zygote (fertilised egg).
Heterozygous: having two different alleles (e.g., $Tt$ )

Diploid and haploid cells

Diploid individuals have two alleles of each gene (one from each parent)
Haploid cells (gametes) have only one allele of each gene

Autosomal recessive inheritance

Mendel's model of inheritance applies under specific conditions, creating what we now call autosomal recessive inheritance. This pattern occurs when certain key conditions are met.

chatImportant

Seven key conditions for autosomal recessive inheritance:

Each characteristic is controlled by a pair of inherited factors (alleles), one from each parent
Alleles pass from generation to generation according to predictable ratios
Alleles in an individual may be the same (homozygous) or different (heterozygous)
In heterozygous individuals, the dominant allele is expressed whilst the recessive allele is masked
A recessive trait only appears when both alleles are recessive
During gamete formation, allele pairs segregate so each gamete receives only one allele
When studying multiple traits, allele pairs separate independently of each other

Notation for alleles

infoNote

Scientists use letters to represent alleles:

Capital letter for the dominant allele (e.g., $T$ for tall)
Lower case letter for the recessive allele (e.g., $t$ for short)

The capital and lower case versions of the same letter always represent dominant and recessive alleles of the same genetic trait.

Mendel's laws

Mendel's first law: dominance and segregation

This fundamental law states that:

Characteristics are determined by factors (alleles) occurring in pairs
Only one member of an allele pair can be present in any gamete (segregation)
Offspring inherit one factor from each parent
When two heterozygous individuals breed, they produce offspring in a ratio of approximately 3:1 (dominant:recessive phenotype)

Mendel's second law: independent assortment

This law explains how variation arises during meiosis:

When individuals with two or more pairs of contrasting characteristics are crossed, different pairs of factors separate independently
For example, crossing tall plants with yellow pods and short plants with green pods produces some tall green offspring and some short yellow offspring
This law assumes genes are located on different chromosomes

Mendel's monohybrid cross

Mendel's classic experiment involved crossing pure-breeding (homozygous) parents with contrasting traits. This elegant experiment demonstrated the fundamental principles of inheritance.

lightbulbExample

Worked Example: Classic Monohybrid Cross

P generation (parents): Pure-bred tall ( $TT$ ) $\times$ Pure-bred short ( $tt$ )

F₁ generation (first filial): All offspring were tall ( $Tt$ ) - heterozygous hybrids

F₂ generation (second filial): When F₁ hybrids were crossed, offspring appeared in a ratio of 3 tall : 1 short

Genotypic ratio: $1TT : 2Tt : 1tt$

Phenotypic ratio: $3$ tall : $1$ short

This can be expressed as probabilities:

75% probability of tall offspring
25% probability of short offspring

infoNote

These ratios are based on probability and require large sample sizes. Mendel obtained ratios close to, but not exactly, 3:1 in his experiments.

Solving genetics problems with Punnett squares

A Punnett square is a problem-solving tool that helps predict the outcomes of genetic crosses. It shows how alleles segregate during gamete formation and the possible combinations during fertilisation.

infoNote

How to construct a Punnett square:

Write the parent genotypes to be crossed
Show how gametes segregate during meiosis
Draw a 3 × 3 grid
Write one set of gametes across the top
Write the other set of gametes down the side
Fill in each box with the resulting genotype combinations
Calculate genotypic and phenotypic ratios

lightbulbExample

Worked Example: Using a Punnett Square

Crossing two heterozygous tall pea plants ( $Tt \times Tt$ ):

Gametes from each parent: $T$ and $t$

Punnett square results:

$TT$ (tall) - 1 offspring
$Tt$ (tall) - 2 offspring
$tt$ (short) - 1 offspring

Genotypic ratio: 1TT : 2Tt : 1tt

Phenotypic ratio: 3 tall : 1 short

Test crosses

Sometimes an organism's genotype cannot be determined from its phenotype alone. For example, a tall pea plant could be either $TT$ or $Tt$ .

A test cross determines whether an organism showing a dominant trait is homozygous dominant or heterozygous. The method involves crossing the organism in question with an organism that is homozygous recessive.

lightbulbExample

Worked Example: Drosophila Wing Length Test Cross

Long wings ( $V$ ) are dominant to short/vestigial wings ( $v$ ).

If the long-winged parent is homozygous ( $VV$ ):

Table

All offspring will have long wings ( $Vv$ ) - 100% long-winged

If the long-winged parent is heterozygous ( $Vv$ ):

Table

Offspring ratio will be 1:1 (50% long-winged $Vv$ : 50% short-winged $vv$ )

Pedigree analysis

A pedigree chart (family tree) traces the inheritance of a particular trait through multiple generations. These charts use standardised symbols to record phenotypes and help determine genotypes.

Standard pedigree symbols

Squares = males
Circles = females
Shaded symbols = individuals expressing the trait being studied
Horizontal lines = marriages/matings
Vertical lines = offspring relationships
Roman numerals = generation numbers (I, II, III)
Arabic numerals = individual numbers within each generation

Constructing a pedigree chart

infoNote

Steps for constructing a pedigree:

Gather phenotypic information about family members over several generations
Use correct symbols to represent males and females
Shade symbols for individuals expressing the trait
Connect family members with appropriate lines
Number generations with Roman numerals
Number individuals within each generation with Arabic numerals

Uses of pedigree charts

Pedigree analysis helps to:

Record how many family members have a trait
Identify the gender of affected individuals
Determine relationships between family members
Identify inheritance patterns (dominant, recessive, etc.)
Assign genotypes where possible
Predict probability of future offspring inheriting the trait

Analysing pedigree charts

To determine genotypes from a pedigree:

Record known genotypes from phenotypes (use underscore _ for unknown alleles)
Analyse crosses using logic
Look for patterns (e.g., do unaffected parents have affected children?)
Make deductions based on Mendelian ratios
Fill in missing genotypes

chatImportant

Key insight: Recessive traits may "skip" a generation because they can be masked in heterozygous individuals. Studying three generations often reveals hidden recessive alleles.

Autosomal inheritance patterns

Type	Features	Examples
Autosomal recessive	- Affected individuals must have two recessive alleles - Affects males and females equally - Often skips generations	- Albinism - Cystic fibrosis - Sickle cell anaemia
Autosomal dominant	- Only one dominant allele needed - Affects males and females equally - Does not skip generations	- Huntington's disease - Polydactyly

Probability in genetics

Genetic ratios are based on probability and become more accurate with larger sample sizes.

For a typical monohybrid cross ( $Tt \times Tt$ ):

Probability of tall offspring = $\frac{3}{4}$ or 75%
Probability of short offspring = $\frac{1}{4}$ or 25%

infoNote

When Mendel studied hundreds of pea plants, his ratios approached (but didn't exactly match) 3:1. For example:

Height: 787 tall : 277 short = 2.84:1
Seed colour: 6022 yellow : 2001 green = 3.01:1

This demonstrates the importance of large sample sizes in genetic studies.

Remember!

bookmarkSummary

Key Points to Remember:

Genotype is the genetic makeup; phenotype is the observable characteristics
Dominant alleles are expressed in the phenotype; recessive alleles are masked in heterozygous individuals
Homozygous means two identical alleles ( $TT$ or $tt$ ); heterozygous means two different alleles ( $Tt$ )
Mendel's monohybrid cross produces a 3:1 phenotypic ratio and 1:2:1 genotypic ratio in the F₂ generation
Punnett squares help predict offspring ratios from genetic crosses
Test crosses determine whether an organism is homozygous dominant or heterozygous
Pedigree charts trace traits through generations and help identify inheritance patterns
Genetic ratios are based on probability and require large sample sizes for accuracy

Genotypes and Inheritance Patterns (HSC SSCE Biology): Revision Notes