Introduction to Genetics Chapter 11 (M) Genetics and Inheritance Genetics the study of heredity...
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Transcript of Introduction to Genetics Chapter 11 (M) Genetics and Inheritance Genetics the study of heredity...
Introduction to Genetics
Chapter 11 (M)
Genetics and InheritanceGenetics the study of
heredity that deals with the transmission of traits or characteristics from one generation to another
Inheritance The reception of traits by transmission from parent to offspring
Prehistoric Times
Little is known when humans first recognized the importance of genetics
The breeding of horses, cattle, and various breeds of dogs began around 8000 B.C. and 1000 B.C.
Plants such as corn, wheat, and rice was cultivated in Mexico around 5000 B.C.
The History Of Genetics
Pythagoras 500 B.C., a Greek philosopher, stated that human life began with male and female fluids
The History Of Genetics
Aristotle furthered this idea and suggested that these fluids, or “semens,” were actually purified blood—therefore, blood must be part of heredity.
•The theory of Homunculus -17th century
•sex cells contained a complete miniature adult, perfect in form
•This statement was popular way into the 18th century
Small individual
One Bizarre Theory
Blending Hypothesis 1800’s This stated that both the genetic material
from the mother and father were blended in order to produce an offspring
Parents Red Flower X Yellow Flower Offspring Orange (offspring all orange)
Exceptions Red yellow ,etc Theory discarded, If blending occurred all
extreme characteristics would disappear from the population
•Considered to be the “father of genetics•Austrian monk, teacher and mathematician•Expt. approach to genetics•Particulate Hypothesis
Gregor Mendel-1800’s
Particulate Hypothesis
Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits
Used pea plants to study this
Why peas? Rapid reproduction
rate Presence of
distinctive traits Closed structure of
flowers (each pea plant has male (stamens) and female (carpal)
sexual organs) allows self-fertilization
Cross-fertilization
One plant is fertilized by another
Terminology
P Generation parents
F1 Generation first filial
F2 Generation second filial
P X P
F1 F1 F1
F2
Alternative forms of a gene which determines a trait.
Allele
Alleles cont.Uppercase (Capital) letters for
dominant traitsLowercase letters for
recessive traitsEx: Tall = T short = t,
expressed in pairs TT, Tt, tt
Phenotype Physical appearance
Genotype Genetic makeupDominant trait that is easily
observedRecessive trait that is often
maskedHomozygous2 alleles for a
trait are identical TT or ttHeterozygous – 2 alleles for a
trait are not identical Tt
Inheritance Follows Rules of Chance
Mendel began experiments to track the inheritance of characters in pea plants
Results led him to formulate several hypotheses
Seven Traits Studied by Mendel
Terminology
P Generation parents
F1 Generation first filial
F2 Generation second filial
P X P
F1 F1 F1
F2
Mendel’s Experiment Crossed pure
purple and a pure white flower (P generation) =F1
generation All F1 plants
(purple) are crossed by self pollination = F2 generation yields ¾ purple and ¼ yellow
Mendel’s Hypotheses
1. There are alternate forms of genes
2. For each character an organism has two alleles
3. Alleles are either dominant or recessive
4. Alleles segregate during formation of gametes
Law of Dominance
When organisms pure for contrasting traits are crossed,all their offspring will show thedominant trait
Probability Fractions or ratios that will predict that
an event will occur
Punnett Square
Diagram which shows the possible outcome of a cross
Monohybrid Cross
Using a single trait – crossing a pure bred Tall (TT) with a pure bred short (tt) plant
Mating 2 heterozygous black (Bb) rabbits
Test Cross• Individual with dominant
phenotype not possible to predict the genotype run a test cross with individual with recessive phenotype to determine the allele
Dominant Phenotype purple flower (genotype PP or Pp)
Recessive Phenotype white flower ( genotype pp)
Test Cross
Law of IndependentAssortment
Each pair of alleles segregates into gametes independently
Independent Assortment Mendel followed the inheritance of two
different characters ( dihybrid cross)The allele for yellow seeds (Y) is dominant
to the allele for green seeds (y).The allele for round seeds (R) is dominant
to the allele for wrinkled seeds (r). He crossed true-breeding plants that
had yellow, round seeds (YYRR) with true-breeding plants that had green, wrinkled seeds (yyrr).
Dihybrid Cross Crosses Involving Two Traits:
Color: Yellow, GreenShape: Round, Wrinkled
Yellow-RoundYellow-WrinkledGreen-RoundGreen-Wrinkled
The Y and R alleles and y and r alleles stay together
F1 offspring would produce yellow, round seeds.
The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross.
If the two pairs of alleles segregate independently of each other
Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts.
These combinations produce four distinct phenotypes in a 9:3:3:1 ratio
Practice problems
Cross a homozygous yellow, homozygous round plant with a green, wrinkled plant
Cross a homozygous yellow, homozygous round plant with a heterozygous yellow, wrinkled plant
Other Patterns of Inheritance
11.3
Variation in Inheritance
Incomplete DominanceCodominance (multiple alleles)
Polygenic inheritance
Incomplete Dominance Both alleles contribute to a
phenotype of a heterozygous individuals to produce a trait not like either parent.
Phenotype intermediate between two pure traits
Ex: Snapdragons, Andalusian chick.
Snapdragon Flower Color Alleles often written as capital letters with
superscriptsex. CRCR (red) x CWCW (white)
Incomplete Dominance revealed in Heterozygous Individual
A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring
Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring.
Codominance (multiple alleles)Two dominant alleles are
expressed at the same timeEx: Roan coat color in horses
CRCR, CWCW, CRCW
CRCR (Red)CRCR (Red)
CWCW (White)
CRCW (Roan)
If a roan cow (RW) is mated to a roan bull (RW), what are the phenotypes of the offspring?
Multiple Alleles Most genes have more than two alleles
in a population The ABO blood groups in humans are
determined by three alleles, IA, IB, and i.Both the IA and IB alleles are dominant to
the i allele The IA and IB alleles are codominant to each
other Because each individual carries two
alleles, there are six possible genotypes and four possible blood types.
ABO Blood Group System
Type A IA IA or IA i Type B IB IB or IB i Type AB IA IB Type O ii
Problems
A man homozygous for type A blood marries a woman who is heterozygous for type B blood. What are the possible genotypes and phenotypes of their children?
A couple has a child with type O blood. If one parent is type O, what are the possible genotypes of the other parent?
Polygenic inheritance When two or more genes effect a
single characteristicFor example, skin color in humans is
controlled by at least three different genes.
Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance.
An AABBCC individual is dark and aabbcc is light
A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades
The range of phenotypes forms a normal distribution.
Importance of Environment
In some cases environment plays an important part in the expression of genes
Ex: Temperature (Siameses cat fur), Nutrition (height & growth of individual)
However human blood type is not influenced by environment.
Influence of Environment
Meiosis
11.4
MeiosisA type of cell division that
produces four cells Each cell hash half the number
of chromosomes as the parent cell.
In animals, meiosis occurs in the sex organs—the testes in males and the ovaries in females
Meiosis Each species has its own
number of chromosomes Humans 46 or 23 pairs Karyotype display of
these 46 chromosomes Homologus
Chromosomes chromosomes making up the pair that carry the genes for the same trait
Homologus Chromosomes
Two general typesAutosomes 22 pairs found both in
male & femaleSex Chromosomes carry the
gene that determines sex indicated by “X” or “Y”
Sex Chromosomes
Female 22 Autosomes + 1 Sex chromosome, XX 1 from father , 1 from motherEggs 1X + 22 Autosomes
MaleXY X from mother, Y from FatherSperm half have 1X+ 22 autosomes,
other half have 1Y+22 autosomes
Diploid and Haploid Cells
Diploid Cells Cells with 2 sets of chromosomes total number of chromosomes diploid # or 2n Humans 2n=46 in somatic cells
Haploid Cells 1n =23 chromosomes in sex cells, sperm & egg (Gametes)
Human Life cycle
Meiosis
Process by which haploid gametes are formed
Alternation of meiosis and fertilization keeps the # of chromosomes same from generation to generation.
Mitosis 2 offspring w/same # of chromosomes
Meiosis Gametes are formed egg & sperm 4 offspring with ½ the # of
chromosomes Exchange of genetic material
crossing over Oogenesis production of egg 1
egg + 3 smaller polar bodies Spermatogenesis production of
sperm 4 sperm cells
Two Distinct parts of Meiosis
Meiosis I
Meiosis II
Assortment of Chromosomes
Crossing Over/ Genetic Recombination
Comparison of Mitosis & Meiosis By three events in Meiosis l
1. Synapsis and crossing over Homologous chromosomes physically connect
and exchange genetic information
2. Separation of homologues At anaphase I of meiosis, homologous pairs
move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids
separate
3. Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous
chromosomes (tetrads) are positioned on the metaphase plates
Comparison of Mitosis & Meiosis