Unit 8 - Genetics

Genetics is a different sort of biology and one that applies the principles of probability and statistics to biology.

Genetics- defn:

What is a gene?

Trait- defn:

Locus

***Since diploid organisms have paired chromosomes every cell has 2 copies of a given gene.

Allele- defn:

The paired alleles may be :

  1. homozygous-
  1. heterozygous-

genotype-

phenotype-

Mendel’s Genetics

Gregor Mendel- worked in 1857 –

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Mendels Laws-

The Law of Dominance-

Dominant traits are designated with capital letters – A, B, D

Recessive traits are designated with lower caseletters – a, b, d

Therefore- AA X aa  all AA trait offspring

The Law of Segregation of Alleles-

AA

Law of Independent Assortment-

The ______ allows us to mathematically work Independent Assortment problems

ProbAB = Prob A x Prob B where A & B are in appropriate order (not BA)

Heads/tails demo-

Prob H= .5Prob T = .5

variety / HH / HT / TH / TT
Probability / .25 / .25 / .25 / .25
% / 25% / 25% / 25% / 25%
Trials
Tally
Calc prob.
#/100

Punnet Squares- named for Sir Reginald Punnett of 1800s

A tool for determining cross outcomes

Sperm 1 / Sperm 2
Egg 1 / S1 xE1 / S2 xE1
Egg 2 / S1 xE2 / S2 xE2

Mendels First Crosses-

Smooth peas x wrinkled peas

Smooth is dominant over wrinkled

Doing a Monohybrid Cross

Monohybrid-

NOTE- *** follow this format when you do all your cross problems to receive full credit

The problem- With guinea pigs black is dominant over brown. Cross a homozygous brown male guinea pig with a homozygous black female. Predict F1 phenotypes and genotypes

Doing a Mendel Cross

Situation- tall pea plants are dominant over short pea plants

P1- cross a homozygous tall pea plant with a homozygous short pea plant

F1 cross- cross two F1 plants to develop an F2

For those that like to memorize there are 5 Possible Monohybrid Crosses

1. homozygous x homozygous (same)

Production Cross- used to build up purebred stocks

2. homozygous dom. X homozygous rec

Outcross- used to test dominance or create mixed genes

  1. heterozygous x heterozygous

“See What Ya Got” Cross- no particular use

  1. heterozygous x dominant homozygous

Back Cross- used to inject desirable dominant traits into a population

5. heterozygous x rec homozygous

Test Cross- used to expose suspected recessive genes

Doing a Dihybrid Cross

When two or more sets of alleles are hybridized you must determine the gametes made before you can actually do the work..

Problem- In budgerigar parakeets the typical green color is normal dominant. It is made by combining blue pigment and yellow pigment in the feathers. Birds that lack blue pigment due to a recessive gene are yellow and called Lutino. Birds that lack yellow pigment do to a second recessive gene are light blue and called Azure. Birds with neither pigment are albino.

Cross an Azure male budgie with a Lutino female and predict F1 results. Assume both parents are heterozygous for the “other” gene

Incomplete Dominance

Defn:

Ex: In Impatiens the flowers come in 3 colors- red, white and pink

When red flowers are crossed with white flowers all the offspring are pink.

Cross 2 Pink flowers and determine F1 phenotypes and genotypes

P cross- RR x rr

r / r
R / Rr
pink / Rr
pink
R / Rr
pink / Rr
pink

F1 genotypes- all Rr

F1 phenotypes – all pink

Codominance and Multiple Alleles-

Defn:

Chain of dominance-

***** Important Example to Know

Human ABO Blood Type

-on the Red Blood Cell there is a protein that is used by the immune system for recognition. Failure to present the right protein means the cells are destroyed.

-The gene for the protein is traditionally named I

-In the history of our species the gene for the protein has mutated twice and now there are 2 dominant genotypes and a single recessive

Io = generic recessive allele

IA = A type protein dominant allele

IB = B type protein dominant allele

Table of ABO Blood Types

Phenotype – blood type / genotypes / Can donate to / Can receive from
O / Io Io
A / IAIA, IA Io
B / IB IB IB Io
AB / IA IB

What about the + and - ?

The positive and negative included in blood types refers to a second gene called the Rhesus Factor or Rh (also called antigen D).

This second protein is either the dominant + (aka- you have the protein) or the recessive - (no Rh protein)

85% of the US is Rh + = ++ or +- so Rh- (--) is relatively uncommon

Problem- Blue Babies- Hemolytic Disease of the Newborn

Graph of Primary and Secondary Immune Response

Genes and Chromosomes

We know that DNA exists in dividing cells in the form of chromosomes.

The actual location of the genes on the chromosomes has recently been mapped out

Since some genes occur on the same chromosome we say they are______

Linkage-defn:

Since linked genes are attached to each other they do not follow the laws of independent assortment.. In making Punnet Squares only ½ the number of gametes is possible in a dihybrid

Crossing over- defn:

Mapping Loci-

The locations of loci on a chromatid are mapped by using crossover frequency as map units

For example=

In crosses of AAbb x aaBB 80% are as expected AaBb (parental types) and 20% are other types (recombinant types)

Therefore A-B distance equals

In crosses with AAcc x aaCC normal parental types = 60% and recombinant types = 40%

Therefore A-C distance equals

In cases with bbcc x BBCC 80% are as expected (parental types) and 20% are other types (recombinant types)

Therefore B-C distance equals

So the arrangement must be:

Sex Chromosomes and Linkage

Thomas Hunt Morgan- the first Drosophila studies

He discovered –

  1. there are two types of chromosomes
  2. autosomes-
  3. sex chromosomes-

female= XX male = XY

X / Y
X / XX female / XY male
X / XX female / XY male
  1. the X and Y chromosomes carry some genes for non-sexual characteristics.
  1. most of the alleles on the Y chromosome are inactive

Morgans White Eyed Fly-

Morgan found a white eyed fly and bred it with wild type female Drosophila. The crosses illustrate the pattern of Sex Linkage

Wild red eye female X white eyed male  red eyed males and females

Therefore :

So the P cross should be =

R / r
R / RR / Rr
r / Rr / rr
r / r
R / Rr / Rr
R / Rr / Rr

RR x rr

With F1 genos all being Rr

Making the F2 go like this-

Rr x Rr

With phenos being 3 red/1 white and no difference between the sexes

But it didn’t do that- only males inherited white eyes

*So we modify the genotypes to indicate the male individuals and we assume the Y gene is shut down

Redo the crosses

r / Y
R / Rr
Red female / RY
Red male
R / Rr
Red female / RY
Red male

P cross- RR x rY

F1=

2 red female / 1 red male /1 white male

R / Y
R / RR
Red female / RY
Red male
r / RR
Red female / rY
white male

F2 cross-

Rr x RY

Phenotypes-

2 red females / 1 red male / 1 white male

Genotypes-

1RR/1Rr/1RY/1rY

Later research found the following about the Sex Chromosomes

  1. occasional mutants occur with extra or missing sex chromosomes

Condition / Genotype / Frequency / Traits
Klinefelter Syndrome
Turner Syndrome
Super Females
Super Males
  1. In females one of the X’s of the pair is shut off-

Single Active X Hypothesis – Mary Lyon-

  1. Sex determination is different in other animals- we use the X-Y system

X-O System -

Z-W system –

Haploid-diploid system-

Patterns of Dominance

1. Normal Dominance (Complete Dominance

  1. Sex Linked Traits (X-Linked)-
  1. Sex Influenced Traits-

Example- Pattern Baldness

Genotypes / B1B1 / B1B2 / B2B2
Male Phenotype
Female Phenotype
  1. Incomplete Dominance-

5. Codominance and Chain of Dominance-

Pleiotropy-

Epistasis-

Ex: flower color in orchids

Polygenic Inheritance-

Defn:

Ex: hair color, skin pigment, height, wool length, milk production, fruit production, intelligence ?

In polygenic cases we assume expression of phenotype is related to the total of dominant genes present.

AABB / AABb / AAbb / Aabb / aabb
darkest / medium / lightest

Example- in humans some people say intelligence is genetic and smart parents have smart children

Lets assume most parents are in the average intelligence range with genotypes AaBb

Cross two random parents and predict the outcome

AaBb x AaBb

Gametes (each)- AB, aB, Ab, ab

AB / aB / Ab / ab
AB
aB
Ab
ab

Summarize the number of dominant genes in the genotypes then plot the results

Number Dominant
0 / 1
1 / 4
2 / 6
3 / 4
4 / 1

Inbreeding-

Outbreeding-

Hybrid vigor-

Ex:

Overdominance-

Ex:

Human Genetic Diseases

There are a variety of human genetic diseases in the population today. We should know what they are and how they are inherited

Eugenics-

Directions of Medical Genetics

Used primarily for family planning today

  1. The Human Genome Project-
  1. Determination of genetic defects in embryos
  2. ultrasound-
  1. amniocentesis-
  2. chorionic villi sampling-
  1. Gene therapy-

Pedigree Charts-

Survey of Human Genetic Diseases

Trisomy 21- Downs Syndrome-

PKU- Phenylketonuria-

Sickle Cell Anemia-

Cystic Fibrosis-

Tay-Sachs Disease-

Huntingtons Chorea-

Galactosemia-

Red/Green Color Blindness-

Hemophilia A

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