Genetics

Mendel Is known as the

Father of Heredity for his study of heredity (how traits are passed from parent to offspring)

The study of heredity is known as genetics

Mendel worked with pea plants

His work is considered good science because*many trials*many different traits*excellent record keeping*results are repeatable

Mendel’s Law of Dominance Prior to Mendel’s work, it was

believed that a baby inherited all of its factors (now called genes) from one parent

Mendel’s experiment proved that offspring inherit two alleles for each trait, one from each parent

His Law of Dominance: of these two inherited alleles, one masks the expression of the other and is known as dominant. The masked gene is called recessive

Mendel’s Experiment Mendel crossed

two homozygous pea plants (one dominant, one recessive)

All of the members of the F1 generation were tall

Mendel predicted that all generations from then on would also be tall

His prediction was based on the assumption at the time that offspring only inherited genes from one parent (not from both parents as we now know to be true)

Mendel’s Experiment Instead, he found

that 25% of the F2 generation were short

This meant that either the trait magically reappeared, or that it was always present, just hidden

This led to the idea of recessive and dominant genes

Vocabulary to Know Gene: a section

of DNA located on a chromosome that contains directions (codes for) a trait

For example, a gene for eye color

Allele: a version of a gene

Your parents will each give you an eye color gene, but the allele you receive from each could be different (brown vs. blue)

Vocabulary to Know Genotype: the

two letter combination that represents the two alleles you have inherited from your parents. Ex: Bb

Dominant: only one dominant allele is required to show the dominant phenotype

Phenotype: the trait that results from this genotype

Bb= phenotype brown

(pheno= physical expression)

Recessive: two recessive alleles are needed to show the recessive phenotype. The presence of one dominant allele will mask the expression of the recessive trait

Vocabulary to Know Homozygous: a

genotype that contains two of the same alleles

For example, BB or bb

You can differentiate by saying homozygous dominant (BB) or homozygous recessive (bb)

Heterozygous: a genotype that contains two different alleles

For example, Bb

Punnett Squares

Punnett squares are used to predict the outcome of genetic crosses

Monohybrid crosses show the inheritance of one trait

Dihybrid crosses show the inheritance of two traits

Punnett Squares Are based on Mendel’s principle of

segregation, which says that during meiosis, alleles separate so that each gamete carries only one allele (original cell had two)

This relates to our last chapter: segregation happens during anaphase, and the products of meiosis are haploid (one copy of each chromosome)

Patterns of Inheritance

Mendelian Genetics (regular dominance)

One allele is dominant over the other Results in two phenotypes only Dominance Ex: pea plants Tall (TT or Tt) & short

(tt)

Patterns of Inheritance

Incomplete dominance aka intermediate inheritance

The dominant gene does not completely mask the recessive gene, resulting in a phenotype midway between the two

Results in three phenotypes For example: Red + white = pink

Patterns of Inheritance Codominance is when two alleles

cannot mask each other and are both expressed

Seen often in the form of stripes or polka dots in flowers

Results in multiple phenotypes Ex: Red + white= red and white

stripes, red and white polka dots Ex: blood groups/blood types

Patterns of Inheritance

Multiple Alleles In these cases, the gene has more

than two forms (alleles) Blood groups/blood types Results in multiple phenotypes

Patterns of Inheritance

Sex linked genes, aka X linked genes Are located only on the X

chromosome Females receive two X’s while males

only receive one X For this reason, males are more

susceptible to recessive diseases located on the X chromosome as they only have one chance to get the protective dominant gene

Patterns of Inheritance

Polygenic traits are phenotypes that are expressed due to a combination of different genes

Eye color, skin color, hair color, height

Results in a large range of phenotypes

Patterns of Inheritance Summary

Name Example

Mendelian Dominance Rr= red

Incomplete Dominance Rr=pink (not red)

Codominance Rr=red and white stripes or polka dots, type AB blood

Multiple Alleles Blood type alleles: A, B, O

Sex Linked Carried on the X onlyHemophilia, colorblindness, male pattern baldness

Polygenic Many genes impact pheno.Height, Hair, skin, eye color

Environmental factors

Many genes are affected by environmental conditions that can turn genes ‘on’ or ‘off’

In animals, temperature impacts many genes (ex: arctic fox fur color)

In humans, environmental factors include:

nutritionexerciseexposure to sunlightinfectionaltitude

In summary…

“The product of a genotype is generally not a single, rigidly defined phenotype, but a range of possibilities influenced by the environment”

P. 217, Biology: Exploring Life

Pedigrees A pedigree is a

genetic family tree

It is used by genetic counselors to analyze the probability of a disease by using known phenotypes to predict genotypes

Common Genetic Disorders

Symptoms Defect

Cystic Fibrosis

Mucus clogs lungs, liver, pancreas

Failure of Cl ion transport mechanism

Sickle Cell Anemia

Painful, poor blood circulation, clots

Abnormal hemoglobin

Tay Sachs Deterioration of CNS in infancy

Defective enzyme hexosaminidase A

PKU Failure of brain to develop in infancy

Defective enzyme phenylalanine hydroxylase

Huntington’s

Gradual brain function loss

Production of brain metabolism inhibitor

MD Wasting away of muscles

Muscles fibers degenerate and atrophy

Chromosomal Mutations

Down Syndrome is caused by a trisomy of chromosome #21

This is due to a nondisjunction of sister chromatids during anaphase of meiosis when they are supposed to separate

Other chromosomal mutations include:

Duplication

Deletion (part of chromosome breaks off)

Inversion (breaks off, reattaches upside down)

Translocation (breaks off, reattaches on another chromosome)

Diagnosis An

amniocentesis takes cells from the developing fetus (dead skin cells floating in the amniotic fluid)

A picture of the chromosomes called a karyotype is taken

Chromosomal abnormalities can be seen (not individual genes)

Karyotypes