Genetics and Heredity_1_ by pptfiles

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									Mendelian Genetics

Gregor Mendel The Father of Genetics
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Monk in Austria Studied Pea plants in the garden at the monastery Developed the laws of inheritance Noticed different characteristics in pea plants
– Ex: round or wrinkled seeds – Observed 7 characteristics or traits

Gregor Mendel
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Between 1856 and 1863, he cultivated and tested around 28,000 pea plants He found that the plants’ offspring retained traits of the parents His work was not recognized until the turn of the 20th century.

Reasons to Use Pea Plant:
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1. Have enclosed structure so they self-fertilize in nature 2. Pea plants have distinct traits 3. Pea plants have a short reproductive cycle (about 90 days)

Particulate Inheritance
Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA

Genetics and Heredity (Important Terms)
Genetics – Scientific Study of Heredity Heredity – Transmission of characteristics from parent to offspring Trait – any characteristic that can be passed from parent to offspring

TRAITS
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Any characteristics that can be passed from parent to offspring
– Pea plant characeristics: – 1. Plant height (tall or short) – 2. Flower Position (axial or terminal) – 3. Pod Color (green or yellow) – 4. Pod appearance (round or wrinkled) – 5. Seed texture (smooth or wrinkled) – 6. Seed color (green or yellow) – 7. Flower Color (purple or white)

Mendel’s studied traits

Mendel’s Peas
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Mendel would pollinate the pea plants by hand to control the plant characteristics Began by growing plants that were pure for a trait.

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Gene – A genetic Factor that controls a trait Allele – Different versions of a gene for the same trait Dominant Allele – A trait that covers up the opposing trait (represented by capital letter) Recessive Allele – A trait that is often masked (represented by Phenotype – The actual appearance of the organism
– Visual observation

Mendel’s Terms:

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Genotype: The genetic makeup of an organism
– Shows both alleles

Genotype & Phenotype in Flowers

Genotype of alleles:
R = red flower r = yellow flower

All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes Phenotypes RR RED Rr RED rr YELLOW

Terms (cont)
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Heterozygous – Two different alleles for a trait
– Ex: Having both alleles (Tt) – Hybrid

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Homozygous – Two identical alleles for a trait
– Ex: Completely dominant (TT) – Ex: Completely recessive (tt) – Pure bred

Terms (cont)
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Monohybrid Cross – cross involving a single trait (ex: flower color) Dihybrid Cross – cross involving two traits (ex: flower color and plant height)

Punnett Square
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Used to help solve genetics problems.

Mendel’s Crosses
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Parental Generation (P1) – Original pure strain for a trait Mendel crossed P1 generation and produced the First Filial (F1) Generation Mendel then allowed the F1 generation to self-pollinate and produce the Second Filial (F2) Generation

Parental Cross

Mendel’s hybridization experiments…
Monohybrid crosses: Parental Generation True-breeding purple flower x True-breeding white flower

F1 generation

All purple flowers (the hybrids)
Allowed F1 offspring to self-fertilize

F2 generation

705 purple 224 white

Mendel’s F1 Cross

F2 Generation: Phenotype:

3 Purple:1White
Genotype:

1 PP: 2Pp: 1pp

Mendel’s Observations:
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Parental Generation – Crossing two pure bred plants (purple with white) F1 generation all showed the same trait (purple) F1 Generation – Crossing two from the F1 Generation F2 Generation – Showed both traits (3 purple: 1 white)

Following the Generations

Cross 2 Pure Plants TT x tt

Results in all Hybrids Tt

Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

Mendel’s Laws:
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1. Law of Dominance – An allele that ALWAYS controls a trait is Dominant
– An allele that is masked is Recessive – A recessive allele can only be expressed if an organism possesses NO dominant allele for that trait

Mendel’s Laws Cont.
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2. Law of Segregation – The pair of alleles are separated during meiosis

Mendel’s Laws cont.
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3. Law of Independent Assortment – The alleles for different characteristics are distributed independently
– Provides variation – EX: blonde hair and blue eyes does not always inherit together

Summary of Mendel’s laws
LAW
DOMINANCE

PARENT CROSS
TT x tt tall x short

OFFSPRING
100% Tt tall

SEGREGATION

Tt x Tt tall x tall
RrGg x RrGg round & green x round & green

75% tall 25% short
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods

INDEPENDENT ASSORTMENT

Probability:
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Likelihood that an event will occur Expressed as decimal, fraction, or percentage P = # times event occurs # opportunities for the event to occur Example: Flipping a coin (heads vs. tails)
• 2 possible outcomes • Chance of getting tails is 1 out of 2 or 1/2

Punnett Squares
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A grid for organizing genetic information
– Shows probabilities not actual results – Predicts chances of passing on a trait

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Monohybrid Cross – Studies one trait at a time Dihybrid Cross – Studies the probability of having two traits passed on together.

Monohybrid Cross

Step 1

Step 2

Step 3

Step 4

Monohybrid Crosses Try your own!!!!

Monohybrid Cross Example 1:
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Homozygous Dominant x Homozygous Recessive PP x pp Phenotypic Ratio:
– 4 Purple

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Genotypic Ratio:
– 4 Pp

Monohybrid Cross Example 2:
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Homozygous Dominant x Heterozygous PP x Pp Phenotypic Ratio:
– 4 Purple

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Genotypic Ratio:
– 2 Pp : 2 PP

Monohybrid Cross Example 3:
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Heterozygous x Heterozygous Pp x Pp Phenotypic Ratio:
– 3Purple : 1 White

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Genotypic Ratio:
– 1 PP : 2 Pp : 1 pp

Monohybrid Cross : TESTCROSS
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Test Cross – Determines whether a particular characteristic of a plant or animal is Homozygous Dominant (pure bred) or Heterozygous (hybrid) Unknown: P_ x pp If results are all Dominant – Unknown was Homozygous If results show both traits – Unknown was heterozygous

Dihybrid Cross:
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Studying two traits….looking at the probability of passing on two traits together.

Dihybrid Cross
RY RY Ry rY ry

Ry
rY ry

Dihybrid Cross
RY RY RRYY Ry RRYy rY RrYY ry RrYy Round/Yellow: Round/green: 9 3

Ry RRYy
rY RrYY ry RrYy

RRyy
RrYy Rryy

RrYy
rrYY rrYy

Rryy
rrYy rryy

wrinkled/Yellow: 3

wrinkled/green:

1

9:3:3:1 phenotypic ratio

Dihybrid Cross
Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1

Incomplete Dominance
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When 2 or more alleles influence the phenotype, resulting in a phenotype intermediate between dominant trait and recessive trait Ex: Some flowers
– Red Flower – RR – White Flower – rr – Pink Flower - Rr

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Incomplete Dominance

CoDominance:
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When both alleles for a gene are expressed in a heterozygous offspring Neither is dominant or recessive
– Ex: Horse color

Codominance
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Example: blood type 1. 2. 3. 4. type A = type B = type AB = type O = IAIA or IAi IBIB or IBi IAIB ii

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Codominance Problem
Example: homozygous male Type B (IBIB)  x heterozygous female Type A (IAi) A
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I

i

IB IB

IAIB IAIB

IBi IBi

1/2 = IAIB 1/2 = IBi

Another Codominance Problem
• Example: male Type O (ii) x female type AB (IAIB)
IA i IAi IB IBi
1/2 = IAi 1/2 = IBi

i

IAi

IBi

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Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

Codominance

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boy - type O (ii) X girl - type AB (IAIB)

Chromosomes and Inheritance
Material of inheritance is carried by Chromosomes

Linkage Groups:
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Genes located on one chromosome 2 or more genes that are found on the same chromosome are said to be linked Linked genes tend to be inherited together because they are so close together on the chromosome Crossing over – Exchange of genes between homologous pairs of chromosomes

What would you expect from a cross between GgLl x GgLl?
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G = grey body, g = black body L = long wing, l = short wing

9:3:3:1 ratio

But…Morgan got different results!
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His results:
– 3 grey-long : 1 black-short – And a few grey-short & black-long

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Why? These characteristics are on the SAME chromosome, thus they are LINKED together during meiosis

How do linked genes get “unlinked”?
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Crossing Over
The frequency of crossing over between certain genes is used to make a chromosome map

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Chromosome Map
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1 map unit = 1% chance of crossing over
a b A B

c

C

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Which two genes have the highest probability of crossing over? The lowest?

a b

A B Highest: A & C

Lowest: A & B

c

C

What makes human males different than females?
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Sex chromosomes (X and Y)
– Male: XY – Female: XX

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Gametes:
– Egg: carry only X – Sperm: carry either X or Y

Who Discovered Sex Chromosomes?
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Thomas Morgan
– Early 1900s – Columbia University (USA) – Worked with fruit flies

Sex Chromosomes
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Humans: Female XX Male XY Sex of a baby is determined by the male
– A sperm with X will produce a female baby – A sperm with Y will produce a male baby

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Some organisms do not have sex chromosomes
– EX: bees and ants – sex is determined by the total # of chromosomes – Ex: Alligators – sex is determined by the temperature of eggs.

Sex-Linked Traits:
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X-Linked genes – genes found on the X chromosome Y-Linked genes – genes found on the Y chromosome
X-linked traits show up more often in males than in females….Females have a chance to cover them up (because they have XX)

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Sex Linked Traits
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Most sex linked genes are found on the X chromosome

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Only genes on the Y chromosome are for male reproductive organ development

Sex-linked Trait Problem
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Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) XRY x X rXr Remember: the Y chromosome in males does not carry traits. Xr Xr RR = red eyed Rr = red eyed XR rr = white eyed XY = male XX = female Y

Sex-linked Trait Solution:

Xr

Xr XR Xr 50% red eyed female 50% white eyed male

XR
Y

XR

Xr

Xr Y

Xr Y

Sex-Limited Traits:
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Autosomal traits that are expressed in only one sex These genes are activated by hormones of one sex but not hormones of the other sex
– EX: beard growth in males

Sex-Influenced Traits
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Expressed in both sexes, but expressed differently Ex: Gene for baldness
– In presence of male sex hormones, the gene is dominant – In presence of female sex hormones, the gene is recessive

Pedigree
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A chart that shows how a trait is inherited Used to follow a specific trait through a family’s lineage Usually used to trace a disease.

Pedigree Key
Normal male Affected male Unmarried Normal female Affected female Marriage

Dead

Pedigree:

Karyotype:
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A photograph that shows an individual’s chromosomes in homologous pairs To get a karyotype – take individual cells (usually white blood cells)
– Cells allowed to begin mitosis but stopped in metaphase….individual chromosomes are isolated, stained, and photographed.

Karyotype

Human Patterns of Inheritance
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Single allele trait Multiple allele trait Polygenic trait X-linked trait Nondisjunction

1. Single Allele Trait
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A trait that is controlled by a single allele of a gene Normal dominant-recessive (Mendel) Example Genetic Disorders:
– Huntington’s Disease (autosomal dominant) – Cystic Fibrosis (autosomal recessive)

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2. Multiple Allele Trait
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3 or more alleles of the same gene code for a single trait Example: ABO Blood Type IA = type A (dominant) IB = type B (dominant) i = type O (recessive)

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3. Polygenic Trait
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Trait that is controlled by 2 or more genes Range of phenotypes
– Influenced by environmental factors too

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Examples: skin color eye color human height

4. X-Linked Trait
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Trait controlled by a gene on the X chromosome Examples: colorblindness (recessive) hemophilia (recessive)

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5. Nondisjunction
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The failure of chromosomes to separate during meiosis resulting in one gamete with too many chromosomes and one gamete with too few chromosomes

Trisomy

Monosomy

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Trisomy: cell with 3 copies of a chromosome (too many chromosomes) Monosomy: cell with 1 copy of a chromosome (too few chromosome) Example Genetic Disorders: Down Syndrome (Tri-21) Klinefelter’s Syndrome (XXY) Turner’s Syndrome (X__)

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Mutations that Lead to Genetic Disorders:
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Mutation: a change in the DNA of an organism Can involve an entire chromosome or a single nucleotide Can lead to genetic disorders

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Mutation Types
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Germ-cell mutation: occurs in the germ cells (gametes)
– Does not affect the organism – Does affect the organism’s offspring

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Somatic-cell mutation: occurs in the organism’s body cells
– Does affect the organism – Does not affect the organism’s offspring

3.

Lethal mutation: causes death, often before birth

Mutation Types Continued
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Chromosome mutation: change in the structure of a chromosome a. Deletion: b. Inversion: c. Translocation: d. Nondisjunction:

Deletion

Inversion

Translocation

Nondisjunction

Mutation Types Continued
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Gene mutation: involves large segments of DNA or a single nucleotide of DNA a. Point mutation: single nucleotide mutation within a codon b. Frame shift mutation: cause the misreading of codons during translation thus making the wrong protein

Detecting Human Genetic Disorders
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Before Pregnancy:
1. 2.

Genetic Screening: Genetic Counseling:

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During Pregnancy:
1.
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Amniocentesis: Chorionic Villi Sampling:

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After Birth:
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Genetic Screening:


								
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