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LM
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Phases of Mitosis
A. Interphase B. ProphaseB. C. Metaphase D. Anaphase E. Telophase
• A somatic cell Is a typical body cell Has 46 chromosomes in a human
Is used for sexual reproduction Usually termed eggs and sperm
Review of Terms
• A gametic cell (gamete)
• Haploid – cell or organism that contains one complete set of chromosomes
• Diploid - cell or organism that contains two complete sets of chromosomes
• Autosome Chromosome that is not directly involved in determining the
sex of an organism
A chromosome that determines whether an individual is male or female
Review of Terms
• Sex chromosome
Female
Somaticcells
Male
44
XY
44
XX
Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.
Meiosis
Meiosis
MEIOSIS I
Sisterchromatidsseparate.
MEIOSIS II
Homologouschromosomesseparate.
INTERPHASE BEFORE MEIOSIS
Sisterchromatids
Duplicated pair ofhomologouschromosomes
Chromosomesduplicate.
Pair of homologouschromosomes indiploid parent cell
Metaphase I
Metaphase II
Recombinant chromosomes
Gametes Recombinant chromosomescombine geneticinformation fromdifferent parents.
Homologous chromatidsexchange correspondingsegments.
Sister chromatids remain joined at theircentromeres.
Prophase Iof meiosis
Duplicated pair ofhomologouschromosomes
Chiasma, site ofcrossing over
Spindlemicrotubule
Crossing overIn crossing over:
•Homologous chromosomes exchange genetic information •Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs
MeiosisHow do we account for genetic variation?
*Independent assortment *Crossing over *Random fertilization
Independent Assortment:
Cross over:
Mitosis and Meiosis
Meiosis I
Abnormal gametes
Gametes
Nondisjunction:Pair of homologouschromosomes failsto separate.
NONDISJUNCTION IN MEIOSIS I
Number of chromosomes
Meiosis II
Nondisjunction:Pair of sisterchromatidsfails to separate.
Abnormal gametes Normal gametes
n n n 1n 1 n – 1n 1
NONDISJUNCTION IN MEIOSIS II
n – 1 n – 1
Errors in Meiosis
Errors in MeiosisDown Syndrome:•Is also called trisomy 21•Is a condition in which an individual has an extra chromosome 21•Affects about one out of every 700 children
Genetics
Genetics is the scientific study of heredity.
Heredity is the transmission of traits from one generation to the next.
Genetics
Gregor Mendel•Worked in the 1860s•Was the first person to analyze patterns of inheritance•Deduced the fundamental principles of genetics
GeneticsRemovedstamensfrom purpleflower.
White
Stamens
Purple
Transferred pollen fromstamens of white flowerto carpel of purpleflower.
Parents(P)
Carpel
Offspring
(F1)
Pollinated carpelmatured into pod.
Planted seedsfrom pod.
Mendel studied garden peas because they:
•Easy to grow
• Come in many readily distinguishable varieties
• Easily manipulated
• Can self-fertilize
Genetics• A character is a heritable
feature that varies among individuals.
• A trait is a variant of a character.
• Each of the characters Mendel studied occurred in two distinct forms.
Genetics
Purple flowers
F1 Generation
White flowers
P Generation(true-breadingparents)
All plants havepurple flowers
F2 Generation
Fertilizationamong F1 plants(F1 F1)
of plants have purple flowers
of plants have white flowers
34
14
Mendel’s hypotheses (to explain his results)
*genes and alleles
1. Alternative versions of genes (alleles) account for variation in inherited characters.
2. For each character, an organism inherits two alleles, one from each parent.
Genetics
3. If two alleles differ, one is dominant, the other recessive
X
Tall Dwarf
P:
DD dd
F1 – all Tall
TallDd
4. The two alleles for each character segregate (separate) during gamete production.
Mendel’s Law of Segregation
Genetics
GeneticsA Punnett Square predicts the results of a genetic cross between individuals of known genotype
D d d
Tall DwarfP: DD ddX
Gamete formation: D
*genotype
*phenotype
*Homozygous
*Heterozygous
D
D
d d
D d D d
D d D d
4/4 are Dd
4/4 are Tall
Dihybrid cross- A genetic cross between two individuals involving two characters
GGWW ggww
Example:
P1
yellow, round
green, wrinkled
XGW GW GW GW
gw
gw
gw
gw
GgWw
Genetics
F1
All yellow, round
F1
All yellow, roundGgWw
F1
All yellow, roundGgWw
X
F2
9/16 yellow, round
3/16 yellow, wrinkled
3/16 green, round
1/16 green, wrinkled
GW Gw gW gw
gW
GW
gw
Gw
9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw1/16 GGww, 2/16 Ggww
1/16 ggWw, 2/16 ggWw
1/16 ggww
Mendelian inheritance is based on probabilityExample- coin toss
*1/2 chance landing heads
*Each toss is an independent event
*Coin toss, just like the distribution of alleles into gametes
*The rule of multiplication – determines the chance that two or more independent events will occur together
½ x ½ = ¼
B
B
b
Bb
b
Fem
ale
gam
etes B B
B
b b b
Male gametes
Formation of sperm
Bb male
Formation of eggs
Bb female
F2 Genotypes
F1 Genotypes
( )
12
14
12
12
12
14
14
14 1
212
Genetics
Female MaleAttachedFree
Third generation(brother and sister)
Second generation(parents, aunts, and uncles)
First generation(grandparents) Ff Ff
FfFF ff fforFf
ff FForFf
Ffff
Ff ff
Genetics: Pedigrees
Human Disorders
F1 Generation
RR rr
Gametes
P Generation
F2 Generation Sperm
Gametes
Red White
R r
RrPink
R r
R r
R
r
RR Rr
rrRr
Eggs12
12
12
12
12
12
Variations in Mendel’s Laws
In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.
Homozygousfor ability to make
LDL receptors
Severe diseaseMild diseaseCell
Normal
LDLreceptor
LDL
Homozygousfor inability to make
LDL receptors
HeterozygousHH Hh hh
GE
NO
TY
PE
PH
EN
OT
YP
E
Hypercholesterolemia•Dangerously high levels of cholesterol in the blood.•Is a human trait that is incompletely dominant.•Heterozygotes have blood cholesterol levels about 2X normal.•Homozygotes have blood cholesterol levels about 5X normal.
Variations in Mendel’s Laws
BloodGroup
(Phenotype)Genotypes Red Blood Cells
O
A
B
AB
ii
IAIB
IBIB
orIBi
IAIA
or IAi
Carbohydrate A
Carbohydrate B
Variations in Mendel’s LawsMultiple Alleles
Variations in Mendel’s Laws– Pleiotropy is the impact of a single gene on more than one
character.
Pleiotropy Multiple traits(e.g., sickle-celldisease)
Singlegene
Variations in Mendel’s Laws
Multiple genes
Polygenicinheritance Single trait
(e.g., skin color)
– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
Variations in Mendel’s Laws
Variations in Mendel’s Laws
F1 Generation
P Generation
F2 Generation Sperm
AABBCC(very dark)
Eggs
aabbcc(very light)
AaBbCcAaBbCc
18
164 64
66415
6420
6415
646 1
64
18181818181818
18
18
18
18
18
18
18
18
Sex Linkage
*Sex chromosomes*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
Any gene located on a sex chromosome is called a sex-linked gene.•Most sex-linked genes are found on the X chromosome.
Sex Linkage
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