Introduction to Genetics Simple Mendelian Traits and Monohybrid Punnett Squares

What is genetics? Genetics is the scientific study of heredity or how traits (such as hair color, eye color, height, and even genetic disorders) pass from parent to offspring What traits do you have in common with your parents? What traits are different? Do you have traits in common with siblings? Grandparents? Why?

Discovering Genetics In the mid-1800s, an Austrian monk named Gregor Mendel studied how traits were passed from one generation to the next using pea plants He noticed that some traits (like height or flower color) appeared in predictable patterns in the next generation. His ideas form the foundation of the principles of heredity, known as Mendelian Genetics

Homologous Pairs and Genetics Genetics depends on the homologous pairs we discussed in Meiosis. Each parent gives one chromosome from each pair to the offspring. Sometimes these pairs are the same, and sometimes they are different (Ex - XX vs XY). The offspring then uses their new pair (one from mom and one from dad) to determine their own traits. In order to study genetics, Mendel created purebreds through self-pollination. This guaranteed that the parent only had one type of trait to pass on (Homologous pairs were the same. Example BOTH purple or BOTH white) After the purebreds were established, Mendel crossbred to create hybrids or individuals that contain a mix of DNA (homologous pairs were different. Example - one purple and one white)

Tracking Traits Mendel studied 7 different traits (specific characteristics), such as seed color or plant height He tracked the traits through several generations and recorded what trait was seen in the offspring. The parent generation was called the P generation The offspring from those parents are called the F1 generation (first filial) The offspring of the F1 were called the F2 etc.

The Results

Mendel’s Conclusions Biological inheritance is determined by factors (genes) that are passed from one generation to the next. Each trait is controlled by one gene occurring in two contrasting forms – the different forms of each gene are called alleles (Example, the gene for plant height has a tall allele and a short allele.)

Mendel’s Conclusions 2. Law of Dominance- Some alleles are dominant and others are recessive. This can be determined by which allele shows in a hybrid individual. Example – a cross between a purebred tall parent and a purebred short parent will give 100% tall offspring. Therefore, the tall allele is dominant and the short allele is recessive. Recessive alleles will be used ONLY if no dominant allele is present.

What happened to the recessive allele? Mendel then crossed the F1 generation (hybrid with hybrid) and noted that the recessive trait reappeared about 25% of the time in the F2 generation. The recessive trait still transfers, but it is hidden by the dominant trait.

Law of Segregation Alleles (found in homologous pairs) are segregated (separated) from each other during meiosis so that each gamete (egg, sperm) carries only a single copy of each gene New pairs form during fertilization

Genetics is NOT guaranteed! We never know which sperm and which egg will be used to create the offspring, so genetics is entirely based on probability (the likelihood that an event will occur.) For example, coin flipping: the probability of flipping heads is ½ (50%) the probability of flipping three heads in a row is ½ x ½ x ½ = 1/8 (1 in 8 chance) For example, gender determination (X or Y sperm?) The probability of getting a boy is ½ (50%) The probability of getting a boy three times in a row is ½ x ½ x ½ = 1/8 (1 in 8 chance) Past outcomes do not affect future ones!

Punnett Squares To determine the likelihood of a specific trait being passed on, a Punnett square is used. This is a diagram to identify all possible combinations of alleles in offspring when two parents are “crossed”.

Two types of alleles In Punnett Squares, DOMINANT alleles are represented by CAPITAL letters ex: T is for tall recessive alleles are represented by the same letter, but in lowercase form ex: t is for short

Dominant or recessive? Which of the following alleles are DOMINANT? A t q R s Which of the following alleles are DOMINANT? A t q R s Which of the following alleles are recessive? Which of the following alleles are recessive? A t q R s

Two types of alleles Each person has 2 alleles in each pair – one from mom, one from dad homozygous means you have 2 of the same allele (either both dominant, TT, or both recessive, tt) also known as purebred heterozygous means you have 2 different alleles (one dominant, one recessive: Tt) also known as hybrid

Homozygous or heterozygous? Label the following as homozygous (homoz) or heterozygous (heteroz): Hh heteroz BB homoz rr homoz Ss heteroz Tt heteroz qq homoz HH homoz Label the following as homozygous (homoz) or heterozygous (heteroz): Hh BB rr Ss Tt qq HH

Genotype and phenotype A Punnett square can be used to predict the genotypes and phenotypes possible in the offspring A phenotype is the physical characteristic (what it looks like) ex: tall or short A genotype is the genetic make-up (the actual alleles) ex: TT, tt, or Tt

Monohybrid Crosses Although you inherit a lot of genes from your parents, monohybrid Punnett squares only look at a single gene or one trait at a time

Monohybrid Crosses Place parents’ genotypes along the top and side. These must be determined through inferences. For example, a parent showing a recessive trait MUST be homozygous recessive. Fill in the square to find the possible offspring genotypes.

Monohybrid Crosses Try It: cross a homozygous tall plant with a homozygous short plant