Simple Mendelian Genetics

Mendelian Genetics

Dominant and Recessive

Gregor Mendel- a monk and mathematician

• known as Father of Genetics

• studied thousands of pea plants and came up with the basic ideas of simple genetics (simple dominance and simple recessive)

• His experiments led him to conclude that each trait was controlled by two factors

Mendel and Modern Genetics

Mendel lived in the 1800s and knew nothing of genetics, yet his findings have been accepted by modern geneticists

Mendel's Pea Plants

• He observed flower colors, seed colors, and seed shapes--- all of which are examples of "traits".

• Traits- characteristics passed on to offspring from parents

Mendel and His Thousands of Pea Plants

• Mendel performed thousands of experiments with the pea plants over many years

• He noticed that certain traits would appear in one generation and then disappear in the next generation

• Then, in later generations of pea plants, the trait that had disappeared would show up again.

Multiple Choice

Which of the following is NOT a trait that Mendel observed in his pea plants?

• seed color

• seed shape

• length of leaves

• flower color

An Example of Disappearing and Reappearing Traits

• Purple versus white flowers

• Generation 1: True-breeding purple crossed with true-breeding purple produced all purple flowers (offspring 1)

• Generation 2: Offspring 1 (all purple flowers) crossed with true-breeding white flowers produced all purple flowers (Offspring 2)

• Generation 3: Offspring 2 self-pollinates (basically reproduces with only itself) and produces mostly purple and some white flowers.

Why did the white flower trait not show up in one generation and then showed up in the next generation?

• Mendel observed this happening over and over and came to the conclusion that the purple was sometimes able to "block" the appearance of the white flowers.

• The purple trait was "dominant" over the white trait.

• The white flower trait was "recessive".

Law of Dominance

• a natural law stating that a dominant allele will always mask a recessive allele

Dominant versus Recessive

• Dominant- a trait that is expressed over another trait

• Recessive- a trait that can be hidden by another trait

• If one trait is dominant and another trait is recessive, and either one appears at different times throughout generations, Mendel concluded that there must be two factors involved in heredity- one factor from each parent.

• Modern genetics calls these factors "alleles".

Multiple Choice

What observation did Mendel make about certain traits that led him to conclude that some traits were dominant to other traits?

• Some traits appeared once every 10 generations.

• Some traits appeared twice every 10 generations.

• Some traits never appeared.

• Some traits appeared in most or all of the generations.

Multiple Choice

Mendel also observed seed color in pea plants. In a study of several generations, yellow seeds appeared more frequently than green seeds. Which of the following is most likely the reason for this observation?

• Green seeds are dominant to yellow seeds.

• Yellow seeds are recessive to green seeds

• Green seeds are recessive to yellow seeds.

Mendelian Genetics in Animals

• There are some traits in animals that follow the pattern of Mendelian genetics.

• Fur color, eye color, length of ears, etc. are all traits that can follow the law of dominance

Modern Mendelian Genetics

• We now know that each trait in an organism is controlled by genes (segments of DNA that code for specific proteins that control specific traits).

• However, a single gene has many different variations within it that provides for diverse options

• Alleles- different variations of the same gene

• There are two alleles for each trait- one from each parent

Possible Alleles

• homozygous (or pure)- having two of the same alleles for a trait 

• heterozygous (or hybrid)- having two different alleles for a trait

• Homozygous dominant- having two dominant alleles for a trait

• Homozygous recessive- having two recessive alleles for a trait

Representing Alleles

• Alphabetic letters represent alleles

• lowercase letters represent "recessive" alleles

• uppercase, or capital, letters represent "dominant" alleles

• 
  

Representing Alleles, continued

• **Recall that there are two alleles for each trait

• two capital letters (ex: TT) are called homozygous dominant alleles

• two lowercase letters (ex: tt) are called homozygous recessive alleles

• an allele pair with one capital letter and one lowercase letter (ex: Tt) are called heterozygous

Multiple Choice

Which of the following is correct?

• Yy is homozygous dominant.

• YY is heterozygous.

• yy is homozygous recessive.

• YY is homozygous recessive.

Multiple Choice

Which of the following represents a heterozygous genotype?

• Aa

• AA

• aa

• Ab

Multiple Choice

A recessive trait would appear in the phenotype of which of these?

• BB

• bb

• Bb

• Bf

Multiple Choice

Which of the following best describes Mendel's law of dominance?

• When an organism has two different alleles of the same gene, both genes will be expressed equally.

• When an organism has two alleles, one of genes will be expressed and the other will be hidden.

• When an organism has two different alleles of the same gene, one will always be dominant.

• The stronger of the two genes will be expressed.

Drag and Drop

The following physical descriptions are ​

Placeholder

hair color, eye color, height.

Drag these tiles and drop them in the correct blank above

phenotypes

genotypes