Mendel's Laws (College Board AP® Biology)

Mendel's Laws

• Gregor Mendel was an Austrian monk

• In the mid-19th century, Mendel carried out breeding experiments on large numbers of pea plants whilst looking after the monastery gardens where he lived

• He studied how characteristics were passed on between generations of plants

• Due to his extensive work on the understanding of inheritance, he is sometimes called the Father of Genetics 

• Mendel artificially pollinated the plants to allow sexual reproduction

  • He did this by carefully transferring pollen (the male gamete) from the anthers of one pea plant to the ovary (female reproductive parts) of another

  • This technique eliminated any uncertainty from his data since he knew which pollen had fertilized each of the plants 

• He collected the pea seeds from these plants and grew them in favorable conditions to find out their characteristics

• He also crossbred offspring peas in order to find out which, if any characteristics would appear in future generations

• Mendel investigated the height of pea plants, the colors of their flowers and the smoothness of their seed coats

Mendel's Breeding Experiments of Pea Plants Diagram

In this cross, Mendel investigated the inheritance of height in pea plants

• Mendel found that characteristics were inherited in a predictable pattern

  • All pea plants in the first generation had the same characteristic as one of the parental plants (e.g. they were all tall)

  • The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio

  • Without knowing it, Mendel had discovered genes, he referred to them as 'units of inheritance'

  • He also discovered that some genes are dominant and some genes are recessive

  • Different forms of the same gene are called alleles

• These observations formed the basis of Gregor Mendel's laws of inheritance

Mendel's First Law - The Law of Segregation

Each trait is determined by two alleles that are randomly separated during gamete formation

• Gametes contain one allele of each gene

  • This means there are two possible allele outcomes for each gene in each gamete

• During fertilization, two gametes fuse to restore the diploid number of alleles - two alleles for each gene

• Combinations of alleles in offspring are random due to random segregation and random fertilization

Mendel's Second Law - The Law of Independent Assortment

Genes located on different chromosomes are assorted independently during gamete formation

• There are 23 pairs of chromosomes and each homologous pair contains a different set of genes

• The inheritance of traits on one chromosome does not influence which traits are inherited on another chromosome

• The alleles of a gene are segregated randomly into different gametes and the assortment of alleles of one gene does not affect the assortment of alleles for another gene

• During fertilization, two gametes fuse to restore the diploid number of alleles - two alleles for each gene

• Combinations of alleles from different genes in offspring are random due to random assortment and random fertilization

Monohybrid Crosses

Monohybrid Inheritance

• The rules of probability can be applied to predict the inheritance of single gene, or monohybrid, traits

• In a monohybrid cross, we consider that the gene has two alleles

  • Where one allele is dominant and the other is recessive

  • Sometimes the alleles are codominant

• To predict the outcome of a monohybrid cross, we start with purebreeding parents (homozygous), each displaying a different phenotype

  • Homozygous dominant and homozygous recessive 

  • This generation is known as the parental generation, denoted as the P generation

• We can use a Punnett grid to predict the probability of a certain offspring, of the P generation, displaying a certain genotype or phenotype

Steps in Constructing a Punnett Grid

1. Write down the parental phenotypes and genotypes

2. Write down all the possible gamete genotypes that each parent could produce for sexual reproduction

   • A useful convention is to write the gamete genotypes inside a circle to denote them as gametes (haploid cells)

3. Place each parental genotype against one axis of a Punnett grid (2 x 2 table)

4. In the boxes of the Punnett grid, combine the gametes into the possible genotypes of the offspring

   • This gives the offspring of the F₁ generation (1^st filial generation)

5. List the phenotype and genotype ratios for the offspring

Dihybrid Crosses

• Monohybrid crosses look at how the alleles of one gene transfer across generations

• Dihybrid crosses look at how the alleles of two genes transfer across generations

  • i.e. dihybrid crosses can be used to show the inheritance of two completely different characteristics in an individual, for example unlinked genes

• The genetic diagrams for both types of cross are very similar

• For dihybrid crosses, there are several more genotypes and phenotypes involved

• When writing out the different genotypes, write the two alleles for one gene, followed immediately by the two alleles for the other gene

• Do not mix up the alleles from the different genes

  • For example, if there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg

• Alleles are usually shown side by side in dihybrid crosses e.g. TtBb

Dihybrid Cross Punnett Square Table

• Predicted ratio of phenotypes in offspring

  • 3 black coat, brown eyes :

  • 3 chestnut coat, brown eyes :

  • 1 black coat, blue eyes :

  • 1 chestnut coat, blue eyes

• Predicted ratio of genotypes in offspring = 3 BbEE : 3 bbEE : 1 Bbee : 1 bbee

Laws of Probability

• Rules of probability (as in the worked example above) can be applied to analyze the inheritance of single gene characteristics from parent to offspring; this is called monohybrid inheritance

If events A and B are mutually exclusive, then:
P(A or B) = P(A) + P(B)

If events A and B are independent, then:
P(A and B) = P(A) × P(B)

• It is important to remember that the rules of probability will only apply accurately to unlinked genes

• However the data yielded by crossing experiments and formation pedigree diagrams (with the genotypes and phenotypes of parents and offspring) can lead to conclusions that support other types of inheritance:

  • Dihybrid inheritance

  • Linked inheritance (genes on the same chromosome in pairs 1 - 22 in humans)

  • Sex-linked inheritance

Genetic Pedigree Diagram

The data contained in a pedigree diagram allows patterns of inheritance to be mapped out and traced to predict the inheritance of genes even if they are linked-genes