Unit 13 - Genetics

Alleles

• different forms of the same gene

• how different? a little bit, maybe even one nucleotide, that’s why they’re considered the same gene just different versions of it

Dominant

trait that is seen as long as there is one dominant allele

Recessive

trait that is only seen in the presence of 2 recessive alleles

Homozygous

having 2 like alleles

Heterozygous

having 2 different alleles

Genotypes

actual gene makeup for a particular locus or trait

Phenotypes

visible trait

Law of Segregation

Alleles separate when gametes form

• When the gametes form – each gamete receives only 1 of each pair of alleles.

Law of Independent Assortment

• If genes aren’t on the same chromosome (linked) they will not have to remain together in the gamete

• if they are linked, they will sometimes assort independently due to crossing over

When do you multiply?

when _ and _ have to happen.

When do you add?

When _ or - have to happen.

Incomplete dominance

Mix

• white + red = pink

• straight + curly = wavy

Codominance

Both exist together

• white cow + red cow = red & white spotted cow

Multiple Alleles

• More than two allele choices although there is always only have 2 alleles at each gene locus

• Example:  Human Blood Types
  Alleles = A, B, & O (also an example of codominance)

What is the most common blood type?

O, even though it’s recessive.

X-inactivation

after embryonic development, 1 X is inactivated in each cell of females only

Barr Body

The inactivated X in a female that is is super-condensed

How does X-inactivation work?

• In each cell, the X inactivated is random

• ~50% of the cells will express the gene on 1 chromosome

• ~50% will express the gene on the other

• overall still expressing both alleles

What causes X-inactivation?

Why does it happen?

• Methylation causes the condensation and turning off of the genes on the X

• Need just one active copy once born (males only have one so if needed both then all males would be abnormal for those traits)

What are pedigrees used for?

• Used to figure out genotypes of family members to see if someone is carrying a disease gene

• Used to determine the mode of inheritance

Pleitropy

one gene affects many traits

• Albinism – 1 mutation in 1 gene affects skin color, eye color, hair color, and eyesight

Polygenic

one trait determined by many genes – continuous pattern (like a gradient of variations)

Multifactorial

may be multiple genes and the environment

Epistasis

When one gene affects the expression of another

• bb – brown coat color

• Bb, BB – black coat color

• C gene – dominant causes color to be deposited in coat so if no C gene then white coat color

Linkage

genes on the same chromosome

Linkage Mapping

• If 2 genes are on the same chromosome, they will remain together in the gamete

  • if they don’t stay together it is due to crossing over

• The closer two genes are on the same chromosome - the more likely they will stay together and not separate due to crossing over

How can you calculate the distance of linked genes?

Calculate % of the time crossing over occurs and separates the genes to tell relatively how far apart they are on the chromosome

• (low % close, high % far)

• Distance isn’t a real # - it’s just relative – called a map unit or Centimorgan.

How can you calculate the distance of linked genes (details)?

• Take one parent that is heterozygous for both genes that you want to map and cross them with another parent that is homozygous recessive – any offspring that aren’t like either parent are due to crossing over

• Take the # of offspring due to crossing over / # of total offspring = % crossing over = relative distance those gene are from each other

Example of Linkage Mapping

• If no crossing over – Blonde Hair and Blue Eyed genes will always end up in the gamete together or Brown Hair and Brown Eyes.

• If crossing over occurs you can get Blonde Hair and Brown eyes in the same gamete etc.

Aneuploidy

abnormal chromosome # (ex. Trisomy)

Polyploidy

3n, 4n (non-disjunction of all chromosomes)

• More normal than aneuploid – some plants live fine but can only reproduce with other polyploid plants

What causes a polyploidy?

• 2n egg and 1n sperm = 3n

Or

• Zygote replicates DNA but doesn’t divide = 4n

Sex Chromosomes and Chromosomal Inheritance

• Non-disjuction of sex chromosomes

• XXY – Klinefelter’s (small testes, sterile, breasts)

• XYY – taller, more aggressive?? Males

• XXX – normal female

• XO – Turner’s Syndrome (no secondary sex characteristics, sterile, short)

Imprinting

• genes that are expressed differently depending on whether they are from the mom or the dad

• the same allele can lead to different traits depending on its parent of origin (sperm vs. egg).

  • In females, although their chromosomes are a mix of maternal and paternal origins, the imprinting on their eggs is reset. All imprinted genes in the egg are marked with a maternal imprinting pattern, even if the mother originally inherited that gene from her father.

  • Many times the methylation turns off the reading of a gene so only the other parent’s allele gets read in the offspring

Example of methylation

Deletion of a gene from chromosome 15

• If inherited from the father:

  • Prader-Willi (retardation, short, obese, small hands and feet, insatiable appetite)

• If inherited from the mother:

  • Angelman’s (uncontrollable laughter, jerky motions, loss of coordination)

What is Fragile X Syndrome and how does it happen?

Fragile X is a disorder where a piece of the X chromosome looks like it's hanging by a thread.

• Caused by too many CGG repeats (normal = ~50, Fragile X = 200+).

• It's the most common inherited cause of intellectual disability.

• The number of repeats can grow over time in the egg, making it worse.

• Worse when inherited from the mother, because the gene gets methylated and turned off.

What is Huntington’s disease, and how is it inherited?

Huntington’s is caused by too many CAG triplet repeats in a gene.

• The number of repeats can get longer (worse) when passed down, especially from the father.

• Note: Mitochondrial DNA is inherited only from the mother, not from the father.

What are some genetic diseases more common in certain groups, and why?

• Cystic Fibrosis (mostly Caucasians):

  • 1 in 25 are carriers; 1 in 2500 have it

  • Messed-up Cl⁻ channel protein → thick mucus, infections

  • Average lifespan ~27 years

• Tay-Sachs (Ashkenazi Jews):

  • Causes seizures, blindness, poor coordination, intellectual disability

• Sickle Cell Disease (African descent):

  • 1 in 10 are carriers

  • Abnormal hemoglobin; gives malaria resistance

• Inbreeding increases the chance of inheriting the same recessive genes, making these diseases more likely in close-knit groups.

What are examples of dominant diseases?

• Achondroplasia (dwarfism):

  • 1 in 10,000

  • Homozygous form is lethal (2 dwarf genes = death)

  • Must be a dwarf to pass it on

  • Two dwarfs can have a normal child

• Huntington’s Disease:

  • Causes nervous system degeneration

What are examples of X-linked diseases?

• Hemophilia

• Color-Blindness
  (More common in males)

What are examples of multifactorial diseases?

• Heart disease

• Diabetes

• Cancer

• Mental illness