Genetics Bio Test

 Genes & Alleles

• DNA’s nitrogenous bases code for genes => Alleles are different versions of homologous genes

  • Every person will have two alleles for every gene - one from each parent

    • Ex: Someone as two blue alleles for their eye color gene resulting in blue eyes

Mendel’s Law

Gregor Mendel’s pea plant experiment resulted in three laws of inheritance

1. Law of Dominance

   1. A dominant allele will express itself over a recessive allele

      1. 2 main types: dominant and recessive

2. Law of Segregation

   1. Genes will segregate equally into gametes - all offspring have an equal likelihood of inheriting either allele from a parent

3. Law of Independent Assortment

   1. The assortment of chromosomes for one gene does not affect the assortment of chromosomes for other genes

Law of Dominance

• Dominant: Allele that will always be physically expressed if present

  • Even if only one out of two

• Recessive: Allele that will only ever be expressed if the dominant allele is NOT present

  • Ex: ABSENCE of a blue eye allele to get blue or green eyes

  • Needs two

Zygosity

• Homozygous: Both of your alleles for a gene are dominant or they are both recessive

  • Homozygous dominant: BB

  • Homozygous recessive: bb

• Heterozygous: You have one dominant and one recessive allele for a gene

  • Heterozygous: Bb

• The only possibility to express the recessive trait is to be homozygous recessive

Genotype vs. Phenotype

• Genotype: The combination of alleles you have for a gene

  • The actual letters => “Bb”

• Phenotype: The physical expression of the gene

  • Brown hair, green eyes, etc.

Generations

• P generation = Parent generation

• F1 generation = First generation of offspring from parents

  • P x P

• F2 generation = Second generation of offspring

  • F1 x F1 = F2

Monohybrid Cross (looking at one specific gene)

• Cross of two parents

  • T = tall

  • t = short

    • Tt x Tt (parent one x parent two)

• Genotypic Ratio => 1:2:1

• Phenotypic Ratio => 3:1 (3 Tall, 1 Short)

PICTURE DETAILS

• All 4 of these possibilities are F1

  • 3 F1’s could be tall bc three have dominant, 1 could be short bc two recessive

• Dominant allele is always written first

Dihybrid Crosses

• Cross of two parents, analyzing two genes at the same time

  • AaBb x AaBb

Dihybrid Axis Inputs

• Dominant - Dominant: at least one big A, and one big B

• If both parents are heterozygous for both genes, the ratio for the phenotypic ratio 9:3:3:1

Dihybrid Cross - (ONLY) Probability (decimal)

• Probability can be used to determine the possibility of a specific combination of genotypes

• Steps:

  • 1. Preform a monohybrid cross for each individual gene and determine the odds of gettin each genotype (fraction)

  • 2. Multiple offs together

  • 3. Answer reflects odds of an individual having ___ trait and ___ trait

  • “AND” = MULTIPLICATION, “OR” = ADDITION

Non-Mendelian Genetics (Complex Inheritance)

Exceptions to Mendel’s Laws

1. Law of Dominance

2. Law of Segregation

3. Law of Independent Assortment

• These are not always true - types of exceptions include …

  • Incomplete dominance

  • Codominance

  • Multiple Alleles

  • Sex-linked

  • Polygenic inheritance

  • Linked genes

Incomplete Dominance

• A heterozygous phenotype is a mixture of the two homozygous phenotypes - neither allele is completely dominant or completely recessive (blended)

Codominance

• A heterozygous phenotype fully and separately expresses both alleles (spotted)

  • No watered down to pink - fully red / white (both colors fully dominant)

  • Ex: Spotted animals

Multiple Alleles

• A and B = dominant

• O = recessive

• Blood donations:

  • O: universal donor (can donate to anybody)

  • AB = can receive from anybody bc they have both

Sex-Linked Traits

• Traits that are passed down on the X or Y chromosome

• X-Linked alleles

  • Recessive

    • People with XX chromosomes need two copies of the allele to express the trait

      • If heterozygous, we consider these people “carriers” - they do not express the trait but have the ability to pass it to offspring

    • People with XY chromosome only need one copy of the allele to express the trait

      • Making the X-Linked traits much more common in males than females

  • X = Does not have trait

  • X’ = Trait Present

  • X’X = Carrier

Polygenic Inheritance

• Some traits are controlled by multiple genes and the combination of alleles on those genes determine phenotype (range)

• Polygenic = range (trait controlled by multiple genes)

• Most things are polygenic inheritance

  • Ex: Height, skin color

Linked Genes

• Some traits are almost always found together. This happens because two genes are incredibly close to each other on the same chromosome making the chances that they will be split up during crossing over very small

  • Red hair and freckles

  • Almost never broken apart during meiosis (bc so close together there is a lesser chance)

    • The further apart two genes are, the more likely they are to break apart during crossing over

Autosomal

NOT sex-linked