Biology Mutations Genetics Guide

What is genetics?

the study of heredity/ how information is passed from parent to offspring

Gregor Mendel

• “Father of Genetics”

• figured out the basics of genetics by breeding pea plants together and observing which traits were present in the offspring

  • He filled up over 3,000 notebooks with observations!

Mendel’s Experiment

P gen

true breeding plants: Parents and ancestors all have the same trait. 

= Homozygous (same alleles)  

trait: the characteristic that you are looking for: i.e. petal color

F₁ Gen

These peas had all purple flowers

hybrids: Parents have different traits 

= Heterozygous (different alleles) 

F₂ Gen

This generation had mixed purple and white flowers, 1 white for every 3 purple

In summary

P - F₁ - F₂ 

Mendel’s hypothesis of inheritance:

- Parents pass on discrete factors (genes) to their offspring 

- These factors are responsible for inherited traits.

1. Each gene has alleles

genes:  “factors” that are passed from parent to offspring

alleles: different versions of each gene

ex. gene = petal color

alleles = purple or white

2.  For each trait, an organism has 2 alleles, one from each parent

Phenotype:  the trait expressed; caused by the genotype

Genotype:  the combination of alleles received from parents 

homozygous: AA or aa if the two alleles from each parent are the same

heterozygous: Aa if the two alleles from each parent are different 

phenotype depends on the genotype
We represent alleles with capital and lowercase letters. i.e.: 

P, p & W, w

Make sure that your capitals are distinguishable from your lowercase letters! 

3.  Some alleles are dominant, and some are recessive.

dominant alleles:  the trait that is expressed over other traits (represented by a capital A)

recessive alleles: traits that can be hidden if a dominant allele is present (represented by a little a)

• A recessive trait is only expressed if they have 2 recessive alleles

4. Principle of Segregation:  

• Each parent has 2 copies of each allele.

• Only one allele gets passed on to each offspring

• They will get another allele from their other parent

5.Principle of Independent Assortment

• The alleles for each trait segregate (separates) independently during gamete formation

• ex. Color and shape are inherited separately

How do we use Mendel’s rules of inheritance to predict traits? 

PROBABILITY AND PUNNETT SQUARES!

How to do a punnett square...

T is the dominant allele for tall plant height

t is the recessive allele for short plant height

Q: Given the following parents:

TT x tt

For the offspring:

What are the % of each genotype?

What are the % of each phenotype?

1. Find genotypes of the parents
   TT x ttPlace one allele of each genotype above/next to each box

2. Bring each allele down and across into each box

3. Write the phenotypes in each box

4.  Figure out % genotype & % phenotype (G-P table)

5. 5.  Figure out % genotype & % phenotype (G-P table)

1: Incomplete Dominance
heterozygous individuals are a blend of the possible alleles

ex.  red flower x white flower = pink flower

             C^RC^R C^WC^W C^RC^W

2.  Codominance
heterozygous individuals show traits of both the possible alleles

(Pandas are cute, but NOT a good example!)

ex.  

black chicken x white chicken = black & white chicken

C^BC^B C^WC^W C^BC^W

3. Multiple Alleles

• a gene that has more than 2 alleles

ex.  human blood type

• possible alleles are I^A, I^B, i

  • I^A and I^B are codominant

  • i is recessive

4.  Polygenic Traits

• traits that are determined by more than 1 gene

ex.  human height is determined by more than 5 genes from each parent; human skin color is another example

5.  Sex-linked traits

• traits determined by genes on the sex chromosomes

  • XX for girls, XY for boys

• sex-linked disorders:  disorders caused by genes on the sex chromosomes - most are on the X chromosome

genes and the environment

• genes provide a range for many characteristics

• where you end up in the range is influenced by your environment

Blood Typing

• antigen:  molecules that act as ‘markers’ on red blood cells, they determine the blood type

• antibodies:  proteins in the blood that will attack foreign blood types ( cause clumping of cells they don’t recognize)

Indicators in a lab to determine blood type:

• “anti-B” serum:  will attack any blood cells with the B antigen (B and AB blood types)

• “anti-A” serum:  will attack blood cells with the A antigen (A and AB blood)

Rh factor:

• Another gene, describing another protein found on the surface of RBC’s

  • + = you have the protein

  • - = you don’t have protein

How does blood typing work?

1. Place 2 small amounts of a blood sample in two separate wells in a spot plate.

2. Add Anti A to one sample. Does it clump?

If yes, the A antigen is present; if no, the A antigen isn’t present.

3. Add Anti B to the other sample. Does it clump?

   1. If yes, the A antigen is present; if no, the A antigen isn’t present.

4. Use what you know about RBC’s to determine the blood type.

1. Gene Disorders

disorder caused by a mutation in a gene

2: Chromosomal Disorders

• Disorders caused by cells with missing or extra chromosomes

• Occurs because of nondisjunction during meiosis

  • ex.  Down’s Syndrome

there are 3 #21 chromosomes

• ex.  Turner’s syndrome

female only gets 1 X chromosome

How do chromosomal disorders occur?

• nondisjunction:  

when chromosomes fail to separate during anaphase in meiosis

Karyotype:

• a picture of a person’s chromosomes

• homologous chromosomes are paired and put in order from 1-23

sex chromosomes: 

the 2 chromosomes that determine gender

autosome: 

the other  22 pairs of chromosomes 

Pedigrees

How to track genetic traits within a family

What is a pedigree?

• a family tree chart that can be used to analyze the pattern of inheritance followed by a specific genetic trait/disorder

Questions to ask a pedigree:

1. Does it occur in every generation? 

2. Does it occur pretty evenly in both males and females?

3. Does every affected individual also have an affected parent? 

How to read a pedigree?

▢ = Male without trait

◼ = Male with trait

◯ = Female without trait

⬤ = Female with trait

◧ ◨ = Male carrier

◐◑ = Female carrier

I, II, III, = generations

I, 1 & 2 are a couple

III, 1 & 2 are siblings

Inheritance patterns:

Autosomal Dominant:  

• only need one mutated allele to get the disease

• disorder usually occurs in every generation 

• occurs in both males and females

• Every affected individual also has an affected parent

ex.  Huntington’s Disease, achondroplasia, Marfans

Autosomal Recessive:  

• need two mutated (recessive) alleles to get the disease

• an affected person usually has parents that are heterozygous carriers

• doesn’t occur every generation

ex.  cystic fibrosis, sickle cell anemia

sex-linked recessive:  

• caused by mutations on the X chromosome

• more common in males

• women are often carriers

ex.  red green colorblindnes