In this lesson we will learn how Gregor Mendel discovered fundamental laws of inheritance by conducting quantitative experiments with garden peas
By the end of this lesson you will be able to:
- Define ‘pure breeding line’ and differentiate between cross-pollination and self-fertilisation.
- Construct Punnett squares for a monohybrid cross and state the genotypes and phenotypes of the F1 and F2 generations, including the Mendelian ratio.
- State Mendel’s conclusions and discuss how they contributed to our current understanding of heredity.
- Describe how a test cross can be used to determine the genotype of an individual with the dominant phenotype.
- Gregor Mendel was an Austrian monk, who performed quantitative studies of inheritance using garden peas in the mid 1800s.
- His findings formed the basis of many aspects of modern genetics, known as Mendelian genetics.
(Image: Unknown, Wikimedia Commons)
- Mendel looked at the inheritance of seven traits in peas, each with two phenotypes.
- Flower colour – purple vs white
- Pea shape – round vs wrinkled
- Pea colour – yellow vs green
- Pod colour – green vs yellow
- Pod shape – inflated vs restricted
- Plant height – tall vs short
- Flower position – axial vs terminal
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Crossing Pure Breeding Lines
- Normally, peas reproduce by self-fertilisation, which, after several generations, results in pure breeding lines.
- A pure breeding line is a group of individuals with the same phenotypes that always produce identical offspring.
- Pure breeding lines have the same genotypes for all genes and are homozygous for all genes.
- Mendel set up experiments involving cross-pollination (crosses) between different pure breeding lines of peas which differed in particular traits.
- For example, he cross-pollinated peas that were identical except for their flower colour.
(Image: ruslanababenko, Pixabay)
- Mendel not only observed the phenotypes of the offspring, but also counted the number of offspring with each phenotype, often involving hundreds of offspring in each experiment.
- He discovered that phenotypes were inherited by discrete ‘factors’ that could not only explain phenotype proportions in offspring, but could also be used to predict phenotype proportions in offspring.
- Mendel’s experiments were conducted at a time before the function of chromosomes was understood and long before the discovery of DNA. We now refer to Mendel’s factors as alleles, which are variants of DNA sequences known as genes, that are carried on chromosomes.
(Image: Mendel Museum)
- To test how phenotypes are transmitted between generations, Mendel set up crosses between pure breeding lines which differed in only one trait.
- This is known as a monohybrid cross.
- For example, consider a monohybrid cross between peas with purple flowers and peas with white flowers.
- When Mendel conducted this cross, he found that all offspring (F1) had purple flowers.
- Mendel then allowed the F1 generation to self-fertilise.
- He found that the next generation (F2) had individuals with purple flowers and individuals with white flowers, in an approximate 3:1 ratio.
- His results are summarised below.
- Mendel made the following conclusions:
- There is no blending of phenotypes; instead, inheritance occurs via ‘discrete factors’. In other words, crossing peas with purple and white flowers did not produce peas with intermediate light-purple flowers.
- This disproved the popular idea at the time of ‘blended inheritance’.
- Phenotypes that are not present in parents can be present in offspring. We see this between the F1 and F2 generations – offspring with white flowers can occur when both parents have purple flowers.
- We now know that the phenotypes Mendel observed are a result of genotypes, which are pairs of alleles that are inherited separately from male and female parents.
- We also know that one type of allele is usually dominant over another, resulting in dominant and recessive phenotypes. Consequently, individuals who are either homozygous dominant or heterozygous will have the dominant phenotype and only individuals who are homozygous recessive will have the recessive phenotype.
- It was fortunate for Mendel that all seven of the traits he observed exhibited dominance and none exhibited incomplete dominance or codominance.
Explaining a Monohybrid Cross Using Punnett Squares
- With our current knowledge about inheritance, the results of Mendel’s monohybrid crosses can be explained using Punnett squares.
- The first stage of a monohybrid cross involves a cross between a homozygous dominant individual (with the dominant phenotype) and a homozygous recessive individual (with the recessive phenotype), as shown below.
- The phenotypes observed in the F1 offspring can be explained by the following Punnett square.
- The homozygous dominant parent only possesses the dominant allele (A) and the homozygous recessive parent only possesses the recessive allele (a).
- All F1 offspring will be heterozygous (Aa).
- All F1 offspring will have the dominant phenotype (purple flowers).
- The second stage of a monohybrid cross involves self-fertilisation of heterozygous individuals (with the dominant phenotype). This is the same as a cross between two heterozygous individuals, as shown below.
- The phenotypes observed in the F2 offspring can be explained by the following Punnett square.
- Each parent possesses both the dominant allele (A) and the recessive allele (a).
- ¼ of F2 offspring will be homozygous dominant (AA).
- ½ of F2 offspring will be heterozygous (Aa).
- ¼ of F2 offspring will be homozygous recessive (aa).
- ¾ of F2 offspring will have the dominant phenotype (purple flowers).
- ¼ of F2 offspring will have the recessive phenotype (white flowers).
- These results explain why Mendel observed a 3:1 dominant-recessive phenotype ratio is all of his monohybrid crosses involving dominant and recessive inheritance.
- This 3:1 phenotype ratio is now known as the Mendelian ratio.
- Since an individual with the dominant phenotype can have two possible genotypes, it is not possible to determine by direct observation whether such an individual is homozygous dominant or heterozygous.
- Mendel devised a simple but powerful procedure called a test cross to determine the genotype of individuals with the dominant phenotype.
- A test cross involves crossing an individual which has the dominant phenotype with an individual which has the recessive phenotype.
- Since the individual with the recessive phenotype can have only one possible genotype (homozygous recessive), a test cross creates two possible scenarios, as shown below with flower colour as an example.
- These scenarios will lead to two different outcomes, as shown by the following Punnett squares.
- If the individual is homozygous dominant (AA):
- All offspring will have the dominant phenotype (purple flowers).
- If the individual is heterozygous (Aa):
- ½ of the offspring will have the dominant phenotype (purple flowers).
- ½ of the offspring will have the recessive phenotype (white flowers).
- Therefore, a test cross can determine the genotype of an individual with the dominant phenotype by observing the phenotypes of the offspring:
- If no offspring have the recessive phenotype, the individual must be homozygous dominant.
- If half (or any) of the offspring have the recessive phenotype, the individual must be heterozygous.
- By conducting quantitative studies of inheritance of several traits in peas, Gregor Mendel developed laws which form the basis of many aspects of modern genetics, known as Mendelian genetics.
- Mendel set up experiments involving crosses between pure breeding lines of peas which differed in particular traits.
- By counting the numbers of each phenotype present in the offspring, Mendel concluded that inheritance occurred by discrete ‘factors’, which we now call alleles.
- Mendel developed a type of cross called a monohybrid cross, which produces two generations of offspring – F1 and F2.
- The first stage involves cross-pollination of two pure breeding lines that differ in one trait.
- This results in all F1 having the dominant phenotype, as they are all heterozygous.
- The second stage involves self-fertilisation of the F1 generation.
- This results in F2 having the dominant and recessive phenotypes in a 3:1 ratio, as ¼ are homozygous dominant, ½ are heterozygous and ¼ are homozygous recessive.
- This 3:1 phenotype ratio is known as the Mendelian ratio.
- Mendel also developed a test cross, which is a simple method for determining the genotype of individuals with the dominant phenotype.
- A test cross involves crossing the individual in question with an individual which has the recessive phenotype.
- If no offspring have the recessive phenotype, the individual in question is homozygous dominant.
- If half (or any) of the offspring have the recessive phenotype, the individual is heterozygous.
(Image: _Alicja_, Pixabay)
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