Unit 5: Heredity

Slogan: Genetic continuity vs. Genetic Variation

Unit Specific notes

Heredity is just DNA transfer

5.1 Meiosis

Vocab ↴

Homologous chromosomes: paired chromosomes with the same gene

Genes: The factors that determine traits that we express

Alleles: The variation of a particular gene

Karyotype: An individual’s collection of chromosomes

Autosomes: Chromosomes 1-22

Sex chromosomes: Specifically chromosomes 23

Life cycle

Fertilization and meiosis alternate in sexual life cycles

Fertilization: combine gametes (sperm + egg)

• Fertilized egg = zygote (2n)

• A zygote divides by mitosis to make a multicellular diploid organism

Meiosis: cell division that reduces the number of chromosomes (2n → n)

• creates gametes

Diploid Vs. Haploid

Diploid cells are given the designation 2n because it had two set of chromosomes.

• Somatic (soma = body) cells are diploid

• ex: humans the 2n is 46

Haploid cells are given the designation n which means there are just one set of chromosomes

• ex: humans n = 23

• ↳ also known as half the chromosomes of the diploid cells

Meiosis

• The process of cell division which produces gametes

• It is necessary for sexual reproduction

• Produces 4 genetically different, haploid cells

  • known as a reduction reaction because it reduces the number of chromosomes by half (2n → n)

How does it differ from mitosis?

Meiosis process overview

• There are two stages in meiosis

  • Meiosis Ⅰ

    • DNA is replicated**

    • Very similar to mitosis

  • Meiosis Ⅱ

    • **NO DNA REPLICATION**

    • Chromosomes number reduction (2n → n)

Meiosis Ⅰ

Interphase Ⅰ:

• Same as mitosis: G1, s, G2

• DNA replication occurs as per usual

• Only one interphase in meiosis

Prophase Ⅰ:

• Chromosomes pairs with homologous chromosomes (synapsis)

• Once they have lined up, crossing over will occur

  • this process results in genetic variety

• Metaphase Ⅰ, anaphase Ⅰ, telophase Ⅰ, and cytokinesis are the same as mitosis

Products of Meiosis Ⅰ:

• 2 diploid cells (2n) with exchanged genetic materials

Meiosis Ⅱ

*DNA does NOT replicate again*

• Each daughter cell from meiosis Ⅰ will under meiosis Ⅱ:

  • Prophase Ⅱ, metaphase Ⅱ, anaphase Ⅱ telophase Ⅱ, and cytokinesis

Products of Meiosis Ⅱ:

• 4 Haploid daughter cells that are genetically different from each other AND parent cells

In humans, females undergo meiosis Ⅰ before they are even born, at puberty hormones trigger the release of one egg per month

• Meiosis Ⅱ doesn’t occur until an egg is fertilized

• Males don’t begin meiosis until puberty and do so until death

Sources of Genetic variation

• Crossing over

  • Exchange genetic material

  • recombinant chromosomes

• Independent assortment of chromosomes

  • Random orientation of homologous pairs in Metaphase Ⅰ

• Random fertilization

  • Any sperm + Any egg

  • 8 million x 8 million = 64 trillion combinations!

Human Chromosomal Disorders

HeLa Cells

• Oldest, most used living cells

• Cervical cancer cells taken from Henrietta Lacks

• “Immortal“ cells

• Used to develop vaccines

Controversy:

• Cells were harvested without her consent

• Genome published without her family’s consent

• Sold for profit, and her family got nothing

5.2 Genetics

Gregor Mendel

• Known as the father of genetics

• Conducted an experiment with pea plants

Mendel’s Laws ↴

• Law of Dominance:

  • some alleles are dominant while others recessive

  • if at least one dominant allele is present, the trait expressed is that of the dominant

• Law of Segregation:

  • During gamete formation, the alleles for each gene segregate from each other

  • This ensures each gamete carriers only one allele for each gene

• Law of Independent Assortment:

  • Genes for different traits can segregate independently during the formation of gametes

Genetics

Heredity: The passing of traits from parent to offspring

Genes: The chemical factors that determine traits

Alleles: Different forms of a gene (T or t)

Dominant alleles are represented by a capital letter (T)

• Dominant is always expressed when at least one dominant allele is present

Recessive alleles are represented by a lowercase (t)

• On expressed when the gene is homozygous recessive (tt)

Homozygous Dominant = TT

Homozygous Recessive = tt

• Both of the above can be referred to as “purebred“ “Wild-type“

heterozygous = Tt

• Can be referred to as a hybrid

*See top for phenotype vs. genotype*

Punnett squares are used to calculate the POSSIBLE outcomes of two parental genes

Test cross ↴

• Used to determine the genotype of the Dominant individual

• Breeds an unknown Dominant (TT or Tt) with a homozygous recessive (tt)

• Based on the results we can deduce what the Dominant individuals genotype was/is

Dihybrid cross ↴

• Involves looking at two genes at once

• Produces a 4×4 Punnett square

• Highkey inefficient to use the Punnett square

• See page 16 Unit 5 notes for mathematic short cut

Non-Mendelian Genetics

• Incomplete dominance: When heterozygous shows an intermediate blended phenotype

  • Red flowers (RR)

  • Pink flowers (Rr)

  • White flowers (rr)

• Codominance: Occurs when there are two or more alleles that are dominant in a phenotype

  • Red (RR)

  • Blue (BB)

  • Red AND blue (RW)

Multiple alleles ↴

lowkey idrk COME BACK TO THIS

Epistasis: When a gene at one locus (location) alters the phenotypic expression of a gene at a second locus

• In mice and other mammals, coat color depends on TWO genes

  • One gene determines the pigment color

  • One gene determines if the color pigment will be deposited in the hair

Polygenic Inheritance: Occurs when some phenotypes determined by additive effects of 2 or more genes

• Examples of polygenetic inheritance are:

  • skin color

  • intelligence

  • weight

  • height

Pleiotropy: When alleles are responsible for multiple phenotypic effects

• Cystic Fibrosis

Lethal Alleles: When expressed allele causes death

• Achondroplasia - Dwarfism

• Huntington’s disease

Sex linked traits

• most sex-linked genes are x-linked genes

• Effects males more than females as they only have one X chromosome

• Females typically tend to be a carrier of sex linked traits

Pedigrees

• A genetic “family tree“ that describes the interrelationships of parents and children across generations

Autosomal Recessive:

• Inheritance may skip generations

• Normal parents produce affected children

• Males and females are effected equally

• Smaller number of affected individuals

Sex-linked Recessive:

• Inheritance skips generations

• Males are affected more often than females

• Females are carriers

5.3 Gene linkage and Mapping Genetics

Chromosome Theory of Inheritance

1. Genes have specific locations on chromosomes

2. Genes on homologous chromosomes segregate away from each other during meiosis

3. Each gene pair segregates independently of other gene pairs

Linked genes: Genes located near each other on the same chromosomes tend to be inherited together

Parental & Recombinant Types

Parental types: Offspring have the same phenotype as one of the two parents

Recombinant Types: Offspring with a new combination of phenotypes

Genetic Recombination: Production of offspring with combination of traits different from either parent

Check pages 25-26 of unit 5 for more info

Linkage Map

• a genetic map that is based on % corssover events

• 1 map unit = 1%

  • By finding the percent recombinant out of offspring, you can determine if a gene is linked

  • 50% and under means it is linked

  • The closer (smaller the %) the less often it will produce recombinants

Kahoot/Additional notes

1 centromere for every 2 chromatids (40 chromatids: 20 centromeres)

Chromatids don’t separate in meiosis 1, they separate in meiosis 2

After 4 cell divisions, 1 zygote turns into 16 cells

multifactorial diseases: genetic + environmental factors contribute

A diploid # of 8 can form 16 combinations of chromosomes

Bacteria doesn’t use meiosis to reproduce (asexual?)

Law of assortment: all genes for traits were on different chromosomes

• based in alignment of tetrads at the equator

Epistasis: multiple genes impact one phenotype

Essential Questions

1. How is DNA transmitted from one cell to another? From one generation to another?

2. How do organisms produce new cells for growth and replacement of dead/dying cells?

3. How do organisms create cells for sexual reproduction? How are those cells different form ones produced for growth and replacement?

4. How do the processes of meiosis, crossing over, and sexual reproduction lead to an increased genetic variety?

5. How did Mendel’s observations shape out understanding of inheritance?

6. What have we learned about inheritance since Mendel’s time? Are inheritance patterns more complex than we once thought?

7. How changes in environment or chromosomal makeup lead to variety in offspring?