Di truyền

Patterns of Gene Inheritance
Chapter 23
Genetics is the study of genes
What is a gene?
A. A factor that controls a heritable characteristic
B. Something on a chromosome
C. Information stored in a segment of DNA
D. Something that encodes a protein

Gregor Mendel
LOTS O’ DEFINITIONS
Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color
Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes
Phenotype—the physical expression of a gene or allele. Ex. Blue eyes
Genotype—the genetic composition of an individual.

MORE DEFINITIONS
Locus—the location of a gene on a chromosome. Plural=loci
Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B
Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b
What is the maximum # of alleles a diploid individual can have at any given locus?
STILL MORE DEFINITIONS!!!
Genotype—the genetic composition of an individual
Homozygous----containing a pair of the same alleles. Can be
Homozygous recessive—two recessive alleles EX. bb, or
Homozygous dominant– two dominant alleles EX BB
Heterozygous—containing two different alleles. EX Bb
Law of Segregation

Each individual has two factors (called genes today) for each trait.
Factors segregate during gametogenesis. Ploidy level of gametes?
Fertilization gives each new individual two factors again.
Ploidy level after fertilization?
Gene locus
Defs
Example--Widow’s Peak
Gametogenesis
Homologous pairs separate during meiosis. When?
 a gamete has only one allele from each pair of alleles.
 If the primary spermatocyte is genotype Ww, the sperm cell would contain either a W or a w, but not both.
Ww represents the genotype of an individual.
Gametes are represented by W or w.
One-Trait Crosses
GENOTYPE?
GENOTYPE?
GENOTYPE?
Punnett Square
EXAMPLE 1—A Monohybrid Cross
Monohybrid cross
The One-Trait Testcross
QUESTION: How can we determine the genotype of a dominant phenotype?
Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).
The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters?
Each pair of alleles segregates independently of the other pairs
All possible combinations of alleles can occur in the gametes
EX: two pairs of alleles segregate independently of each other.
When will different alleles not sort independently?

Telophase II
Independent Assortment
Alignment of Homologs at Metaphase I
Segregation and Independent Assortment
MEIOSIS & GENETIC VARIATION
Independent Assortment

Diploid organisms can produce 2n diff. gametes

Ea. homologous pair can orient in two different ways

Humans: 223 = 8,388,608
(est. 8.4 million)



Each homologous pair can orient in either of two ways

HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT
Gene for brown
eyes
Eye color
Gene for blue
eyes
Gene for black hair
r
Gene for red hair
Hair color
INDEPENDENT ASSORTMENT
OR
During meiosis I, tetrads can line up 2n different ways.
Brown eyes
Black hair
Blue eyes
Red hair
Brown eyes
Red hair
Blue eyes
Black hair
Meiosis I & II
Linked Genes and Independent Assortment
Alignment of Homologs at Metaphase I
L
l
l
L
L
What are the possible genotypes of this individuals gametes?
Dihybrid cross
Probabilities
Two-trait Testcross-How?
Genetic Disorders
Pedigree charts- show patterns of inheritance
Autosomal Dominant Disorders
Autosomes---Non-sex chromosomes
Dominant allele
when under what genotype(s)will an individual show the disorder?
Examples: Autosomal Dominant Disorder
Neurofibromatosis
Small benign tumors,
Gene on chromosome 17.
Huntington Disease
Progressive degeneration of the nervous system
Normal until middle age
No treatment
Gene has been isolated (chromosome 4), genetic testing can diagnose

Huntington disease
Autosomal recessive pedigree chart
Autosomes---Non-sex chromosomes
Recessive allele
when under what genotype(s)will an individual show the disorder?
Autosomal Recessive Disorders
Tay-Sachs Disease
Common among United States Jews of central and eastern European descent.
Death by the age of three or four.
Defective enzyme in lysosomes.
Cystic Fibrosis
Most common lethal genetic disorder among Caucasians
Defective chloride ion transport protein
Osmotic imbalance results in thick mucous in lungs and pancreatic ducts
Cystic fibrosis therapy
Phenylketonuria (PKU)
allele on chromosome 12.
lack an enzyme needed for metabolism of phenylalanine (an amino acid)
Urine test diagnostic.
Brain damage unless controlled by diet.
Polygenic (Quantitative) Traits
Governed by more than one gene pair.
Several genes determine the phenotype.
Produce bell-shaped curve.
EX: Skin color
Polygenic Inheritance
EX 2 Polygenic Inheritance :
Polygenic Disorders
Examples: cancer, schizophrenia, hypertension, diabetes, etc
several genes involved
also environmental influences.

Multiple Allelic Traits
Often more than two alleles exist for a particular gene locus.
Maximum # of alleles at any given locus?
Each individual inherits only two alleles for these genes!!!
EX: Human Blood type
Inheritance of blood type
Details
Incompletely Dominant Traits
Codominance--both alleles equally expressed in a heterozygote. EX ABO Blood type
Incomplete dominance--heterozygote shows intermediate phenotype, representing a blending of traits. EX Wavy hair in Caucasians
Phenotypic ratio ?
1 : 2 : 1.
Incomplete dominance
Example 2-Incomplete Dominance
Sickle-Cell Disease
Caused by incompletely dominant alleles.
Single nucleotide mutation causes abnormal hemoglobin.
HbA = normal hemoglobin
HbS = sickled condition.
Genotype of normal?
Genotype of sickled homozygote?
Genotype of intermediate phenotype?

Sickle-Cell Disease
Molecular Explanation-Dominance
Simple Dominance
One-half the amount of gene product sufficient for phenotype
Incomplete Dominance
Recessive allele not expressed in heterozygote
Homozygous dominant –2 doses, full phenotype
Heterozygote—1 dose, reduced phenotype
Codominant
Both alleles expressed, combined phenotype
Let’s Get Small!!!
Everything’s Relative
WHAT’S HAPPENING??
SUMMARY
Physical Basis of Genes
Lots o’ Definitions
Law of Segregation
Law of Independent Assortment
One-Trait Crosses
Law of Independent Assortment
Two-Trait Crosses
Genetic disorders
Polygenic Traits
Incomplete Dominance/Codominance
REMEMBER!!!
Relate genetics to meiosis
For genetics problems—first try to figure the genotype(s) of the gametes