Biology IB HL - 3.4 Inheritance Part 1

Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants

First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics

Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics

These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)

When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed

E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants

When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1

E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1

Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)

Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)

Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)

Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm

For each factor, one version is dominant over another and will be completely expressed if present

Law of Segregation

Law of Independent Assortment

Principle of Dominance

Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene

The segregation of alleles for one gene occurs independently to that of any other gene*

Recessive alleles will be masked by dominant alleles†

The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)

Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance

Gametes are haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova

During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division

As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs

Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene

Gametes are haploid, meaning they only possess one allele for each gene

When male and female gametes fuse during fertilisation, the resulting zygote will contain two alleles for each gene

Males have only one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY)

If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene

If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene

Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene

The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype

The genotype of a particular gene will typically be either homozygous or heterozygous

The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype

The phenotype is determined by both the genotype and environmental influences

Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other

The dominant allele will mask the recessive allele when in a heterozygous state

Homozygous dominant and heterozygous forms will be phenotypically indistinguishable

The recessive allele will only be expressed in the phenotype when in a homozygous state

When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele

An example of this mode of inheritance is mouse coat colour – black coats (BB or Bb) are dominant to brown coats (bb)

Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual

Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect

When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case)

An example of co-dominance is feathering in chickens – black (CB) and white (CW) feathers create a speckled coat (CBCW)

Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen)

The ABO blood groups are controlled by a single gene with multiple alleles (A, B, O)

The A, B and O alleles all produce a basic antigen on the surface of red blood cells

The A and B alleles are co-dominant and each modify the structure of the antigen to produce different variants

The O allele is recessive and does not modify the basic antigenic structure

When representing blood group alleles, the letter I is used to represent the different antigenic forms (isoantigens)

A allele = IA ; B allele = IB ; O allele = i (recessive)

As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups

AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)

A blood groups cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)

B blood groups cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)

O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)