basic genetics for students

8/3/2019 Basic Genetics for Students

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LAELEL


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OIATDMNN


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EEOYOSHTRZGU


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HNTPPEOYE


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1. Designate letters which will represent thegenes/traits

2. Write down the genotypes (genes) of each

parent3. List the genes that each parent can

contribute.

4. Draw a Punnett square - 4 small squares inthe shape of a window.


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5. Fill in each box of the Punnett square bytransferring the letter above and in front of

each box into each appropriate box. As a

general rule, the capital letter goes first and alowercase letter follows.

6. List the possible genotypes and phenotypes of

the offspring for this cross.


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Capital letters represent dominant

traits, and lowercase letters

represent recessive traits.

T= tall t = short


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Father - TT

Mother - tt


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Genetic Information

Gene basic unit of geneticinformation. Genes determine theinherited characters.

Genome the collection ofgenetic information.

Chromosomes storage units ofgenes.

DNADNA - is a nucleic acid thatcontains the genetic instructionsspecifying the biologicaldevelopment of all cellular formsof life


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Chromosome Logical Structure

Locus location of a gene/markeron the chromosome.

Allele one variant form of agene/marker at a particular locus.

Locus1

Possible Alleles: A1,A2

Locus2

Possible Alleles: B1,B2,B3


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Human Genome

Most human cellscontain 46 chromosomes:

2 sex chromosomes (X,Y):XYin males.XXin females.

22 pairs of chromosomesnamed autosomes.


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Genotypes Phenotypes

At each locus (except for sex chromosomes)

there are 2 genes. These constitute the

individuals genotype at the locus.

The expression of a genotype is termed a

phenotype. For example, hair color, weight,or the presence or absence of a disease.


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Genotypes Phenotypes (example)

Eb- dominant allele.

Ew- recessive allele.

genotypes

phenotypes


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Dominant vs. Recessive

A dominantallele is expressed even if it is

paired with a recessive allele.

A recessive allele is only visible whenpaired with another recessive allele.


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One Locus Inheritance

heterozygote homozygote

21A | A a | a

A | a 3 4 a | a

A | a5 6 a | a

Male

Female


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Two members of a gene pair segregate from each other into

the gametes, so half the gametes carry one member of the

pair and the other half carry the other member of the pair.

Mendels 2nd Law

Y / y y / y

y/y

Y/y

y

Y

all yGamete

production

Gamete

production


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Calculating Probabilities

We want to predictpatterns of inheritanceof traits and diseases in

pedigrees.

E.g., we want to knowthe likelihood that a

dog chosen at randomfrom the studypopulation will have blueeyes.


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X-linked Inheritance

Different results obtained from

reciprocal crosses between red-

eyed and white-eyed

Drosophila.

: The geneExplanation

responsible for eye-color

is X-linked. Females have

2 X-chromosomes, whilemales have 1 X-chromosome

and 1 Y-chromosome.


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Mendels 3rd LawIndependent Assortment

Different gene pairs assort independentlyin gamete formation.

Gene pairs on SEPARATE CHROMOSOMESassort independently at meiosis.

This law is true only in some cases.


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Medical Genetics

When studying rare disorders, 6 generalpatterns of inheritance are observed:

Autosomal recessive

Autosomal dominant X-linked recessive X-linked dominant Codominant

Mitochondrial


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Medical Genetics (cont.)

Autosomal recessive

The disease appears

in male and femalechildren ofunaffected parents.

e.g., cystic fibrosis


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Autosomal dominant

Affected males and

females appear in eachgeneration of thepedigree.

Affected mothers andfathers transmit the

phenotype to both sonsand daughters.

e.g., Huntington disease.


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X-linked recessive

Many more males thanfemales show the disorder.

All the daughters of anaffected male arecarriers.

None of the sons of anaffected male show the

disorder or are carriers. e.g., hemophilia


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X-linkeddominant

Affected males pass thedisorder to all daughters

but to none of their sons. Affected heterozygous

females married tounaffected males pass thecondition to half their sons

and daughters e.g. fragile X syndrome


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Codominant inheritance

Two different versions

(alleles) of a gene can beexpressed, and eachversion makes a slightlydifferent protein

Both alleles influence thegenetic trait or determinethe characteristics of the

genetic condition. E.g. ABO locus


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Mitochondrial inheritance This type of inheritance

applies to genes in

mitochondrialD

NA

Mitochondrial disorderscan appear in everygeneration of a family andcan affect both males andfemales, but fathers do not

pass mitochondrial traitsto their children. E.g. Leber's hereditary

optic neuropathy (LHON)


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Example:

Four oclock plant

White , Red and Pink


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SpongeBob SquarePants recentlymet SpongeSusie Roundpants at adance. SpongeBob is heterozygousfor his square shape, butSpongeSusie is round. Square shape

is dominant to round. Create aPunnett square to show thepossibilities that would result if SpongeBob and SpongeSusie had

children. List the possible genotypesand phenotypes for their children.


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TT - square shape

Tt - square shape,

and tt - round

shape

Phenotypic Ratio:

Square shape-2Round shape -2 or

2:2

Genotypic Ratio:

Tt 2 and tt 2 or

2:2

T t

t Tt ttt Tt tt


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A horizontal line between two symbolsrepresents a mating .

The offspring are connected to each other bya horizontal line above the symbols and to theparents by vertical lines.

Roman numerals (I, II, III, etc.) symbolizegenerations. Arabic numerals (1,2,3, etc.)symbolize birth order within each generation.


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basic genetics for students

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