Bio 106 LECTURE 3

Part 1

Gene Segregation & Integration

GENE SEGREGATION & INTEGRATION

A. Law of Segregation

• B. Law of Independent Assortment

• C. Segregation and Assortment in Haploid Organisms

• D. Dominance Relationship

• E. Multiple Alleles

• F. Lethal Genes

• G. Modifier Genes

• H. Gene interactions

• I. Pseudoalleles

• J. Environmental Influence

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What is a GENE?

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A GENE is an operational region of the chromosomal DNA, part of which can be transcribed into a

functional RNA at the correct time and place during development. Thus,

the gene is composed of the transcribed region and adjacent

regulatory regions (Griffiths et al.)

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Typical Eukaryotic Gene

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LOCUS - Position on a

chromosome where a gene

is located

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ALLELE – any 2 or more related genes of a trait

DOMINANT allele: expresses its effect over another allele; masks the recessive allele in the heterozygous organism

RECESSIVE allele: masked in a heterozygous individual by the presence of dominant allele

DOMINANT vs. RECESSIVE

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IMPORTANT!!! Dominance does not determine the relative FREQUENCY of a gene in a population.

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HOMOZYGOUS vs. HETEROZYGOUS

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GENOTYPE: the genetic constitution

PHENOTYPE: visible

appearance

GENOTYPE GENOTYPE PHENOTYPE

PP Homozygous

purple

Purple

Pp Heterozygous

purple

Purple

pp Homozygous

white

white

GENOTYPE vs. PHENOTYPE

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Mendel’s peas

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DEFINE THE FOLLOWING:

P: parents

F1: first filial generation

F2: 2nd filial generation

Self pollination

Cross pollination

True-breeding

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Law of Segregation

MONOHYBRID CROSS – a cross between 2 individuals involving

1 character

Allele pairs separate and each goes into a different gamete.

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Punnett Square

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Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traits. All possible combinations of factors can occur in the gametes.

Alleles from one locus segregate into gametes independently of those from another locus

OR

DIHYBRID CROSS - A cross between 2 individuals involving 2 characters

PHENOTYPES: 9 Tall plant, green pod: T_G_ 3 Tall plant, yellow pod: T_gg 3 Short plant, green pod: ttG_ 1 short plant, yellow pod: ttgg

GENOTYPES: TTGG (homozygous tall, homozygous green) = 1 TTGg (homozygous tall, heterozygous green) = 2 TTgg (homozygous tall, homozygous yellow) = 1 TtGG (heterozygous tall, homozygous green) = 2 TtGg (heterozygous tall, heterozygous green) = 4 Ttgg (heterozygous tall, homozygous yellow) = 2 ttGG (homozygous short, homozygous green) = 1 ttGg (homozygous short, heterozygous green) = 2 ttgg (homozygous short, homozygous yellow) = 1

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Law of Independent Assortment

9 Y_R_ 3 Y_rr 3 yyR_ 1 yyrr

YYRR

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Forked-line method

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(another option in place of Punnett square)

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DOMINANCE RELATIONSHIP

• Complete Dominance

• Incomplete or No Dominance

• Co-dominance

• Overdominance

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• One allele completely masks the other

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Complete dominance in Mendel’s garden peas

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the heterozygote displays a phenotype

that is an intermediate between the phenotypes

of both homozygote parents

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impatiens

snapdragon

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occurs when the phenotypes

of both parents are

simultaneously expressed in

the same offspring

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occurs when a heterozygote has a more extreme phenotype than that of either of its parents

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Question: Could a child with blood type O be produced from parents with blood types A and B?

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/AO /BO

OO / AA/ BB / AB

P

F

P F

P F

P F

Question: A mother and her child have blood type O. Which blood group can the father NOT belong to?

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/AO /BO

OO / AA/ BB / AB

ABO blood group

Bombay phenotype

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• ABO blood groups: Bombay phenotype

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• ABO blood groups: Bombay phenotype

The ABO blood types are based on the presence or absence of specific sugar residues on the surface of RBC. Almost all individuals possess molecules called the H substance on their RBC.

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• ABO blood groups: Bombay phenotype

Individuals with the IA allele express an enzyme that attaches the sugar N-acetylglucosamine (AcGalNH) to the H substance. The addition of the terminal AcGalNH on the H substance produces the A antigen and this antigen on the surface of RBC is the basis of the type A phenotype.

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• ABO blood groups: Bombay phenotype

Individuals with the IB allele express an enzyme that attaches the sugar galactose to the H substance. The presence of a terminal Gal results in the B antigen on RBC and the type B phenotype.

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• ABO blood groups: Bombay phenotype

Individuals with the IOIO

allele have a mutation in the I gene that results in a nonfunctional protein. Thus individuals who are homozygous for the IO allele lack an enzyme to modify the H substance. - only have the H substance on their RBC (test negative for either A or B antigens) They are blood type O.

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• ABO blood groups: Bombay phenotype

The formation of the H substance requires the H gene. H gene produces an enzyme that adds a sugar residue (fucose) to the H substance precursor, forming the complete H substance. Most people have the dominant H allele of this gene, which codes for the functional enzyme.

Some people have the recessive h allele which does not code for a functional enzyme. Thus hh people cannot produce the H substance.

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• ABO blood groups: Bombay phenotype

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• Predicting the ABO blood type of children based on the phenotypes of their parents should be approached with caution.

• There is a third antigen (H) on the surface of red cells that can prevent the expected ABO blood type from occurring.

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type O blood child can also be born to parents who do not have the O allele if a

recessive form of the allele for the H antigen (or h) also is inherited

from both parents.

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• ABO blood groups: Bombay phenotype

Final phenotypic ratio = 3A : 6AB : 3B : 4O

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WHO is the father?

Given: Monica’s child is type O

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Alleles that cause an organism to die only when present in homozygous condition, where the gene involved is an essential gene.

Fully dominant lethal allele: the carrier is killed in both homozygous and heterozygous conditions.

Completely lethal genes usually cause death

…of the zygote

…later in the embryonic development or

…even after birth or hatching.

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Recessive lethal:

Curly wings, Plum eyes, Stubble bristle in Drosophila melanogaster

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gene for tail in Manx cat

Recessive lethal:

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Recessive lethal:

Tay-Sachs disease Cystic fibrosis Sickle cell anemia

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Dominant

lethal

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Lethal Genes ( (AD)

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Inheritance of Huntington’s disease

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Conditional lethal

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Some lethal alleles exert their effects only under certain environmental conditions.

Some developing Drosophila larvae are killed at 30 C. Larvae will survive if grown at 22 C.

EXAMPLE: ts lethals

Conditional lethal

Favism

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Semi-lethal or sublethal

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