Intro to Genetics Notes Summary & Study Notes

🧬 Intro to Genetics

Genetics is the study of heredity, or how physical traits are passed from generation to generation. A gene is a small section of a chromosome that codes for one protein, and humans have about $30{,}000$ genes. Alleles are different versions of a gene that can influence trait variation.

🧬 Genes and Alleles

A Gene is the basic unit of heredity. An Allele is a version of that gene. For example, the gene for eye color has alleles such as Brown, blue, and green. A second example is the gene for the number of fingers, with alleles like 5 fingers and 6 fingers.

Think: How many alleles do you have for each trait? In most cases, you have two (one from each parent). Think: How many alleles does a reproductive cell carry for each trait? One (you pass on half your DNA to offspring).

🧬 Genetics Vocabulary

Heredity = the passing on of physical characteristics from one generation to another. Genotype: the genes or alleles that make up an individual; what is on the chromosome. Examples: $TT$, $Bb$, $cc$.

Phenotype: the physical appearance of an individual. Examples: tall, brown eyes.

Memory tip: Phenotype – photograph. The phenotype is what you see; the genotype is the genetic makeup you cannot directly see.

Dominant: an allele that is always expressed (you only need one copy to show the trait). Examples: $B$ or $D$. Wild type allele often produces a functional protein; Mutant allele may produce a malfunctioning protein.

Dominant traits include things like Polydactyly or Dark eyes.

Recessive: the trait is shown only when two copies of the allele are present (hidden by a dominant allele). Examples: $b$ or $d$. Recessive traits include Blue eyes or Red hair (red hair is typically a result of two recessive genes).

Homozygous: two alleles for a trait are the same (purebred). Examples: RR (homozygous dominant), rr (homozygous recessive). Other examples with letters: FF, qq, mm – all same.

Heterozygous / Hybrid: two alleles for a trait are different (hybrid). Examples: Rr, Ff, Qq, Mm.

🧬 Gametes and Meiosis

A gamete is a sex cell (sperm or egg). The process that creates gametes is Meiosis. For most traits, an individual has two genes or alleles, and a gamete carries one copy of each gene.

🧬 Punnett Squares

A Punnett Square is a chart that shows the probability of different offspring genotypes from parents. The outside letters represent the parents’ gametes, and the inside boxes show potential offspring.

🧬 Monohybrid Cross Practice

Guinea pigs are often used in genetics because they reproduce rapidly and show clear traits. In the example, black hair (B) is dominant and white hair (b) is recessive.

Trait discussed: hair color in guinea pigs. Dominant form: black ($B$). Recessive form: white ($b$). Dominant allele representation: $B$. Recessive allele representation: $b$.

Example cross: Bb x Bb

• Gametes: $B$ and $b$ from each parent.
• Offspring genotypes: 1 BB : 2 Bb : 1 bb.
• Phenotypes: 3 black : 1 white.

What is the trait discussed in the above problem? hair color What is the dominant form of the trait? black The recessive form? white How would you represent the dominant allele? $B$ The recessive allele? $b$

Dihybrid Crosses What is in store today? Agenda: warm-up vocabulary review, review cell cycle unit, Mendelian genetics basics, introduce dihybrid crosses, practice problems, finish baby lab.

Learning targets: I will be able to apply the rules of segregation and independent assortment to find all of the gamete possibilities for a dihybrid cross.

Vocabulary Review: use the picture to describe genotype, phenotype, and homologous chromosomes. Important to know: P = parental, F1 = first generation, F2 = second generation. Testcross: a cross used to determine genotype of an unknown dominant phenotype by crossing it with a homozygous recessive.

Testcross recap: A yellow seeded plant is crossed with a green seeded plant (green is recessive). Parent genotypes: ___ x ___ . Determine the phenotypic ratios: ___ yellow : ___ green. Set up the square by considering what heterozygous and homozygous mean. Genotype–phenotype mapping: YY = yellow, Yy = yellow, yy = green.

Crossing two heterozygotes for two traits (Aabb, AaBB, AaBb, etc.) can be solved by two methods:

1. Science way: Punnett squares with 16 boxes using the foil method to generate all gametes.
2. Math way: Rule of multiplication using the monohybrid cross results.

Dihybrid crosses example: If the other parent is also heterozygous for both traits, what are the gamete possibilities? YyRr x YyRr yields gametes: YR, Yr, yR, yr.

For dihybrid crosses, you can use the science method with Punnett squares or the math method based on multiplying single-trait probabilities. The Amoeba Sisters video is a recommended resource for visualization.