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Patterns of Inheritance
The science of genetics has ancient roots
(400 BCE) An early explanation for inheritance suggested that particles called pangenes came from all parts of the organism and were incorporated into eggs or sperm to be passed to offspring.
(1800s)The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was suggested but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.
Experimental genetics began in an abbey garden (mid 1800s)
Heredity is the transmission of traits from one generation to the next.
Genetics is the scientific study of heredity.
Gregor Mendel (1860s)
– Father of modern genetics
– Experimented with garden peas
– Found patterns to inheritance of traits
– Inheritance could be predicted
Experimental genetics began in an abbey garden
Mendel used pea plants. Why are they a good organism for genetic studies?
Mendel found evidence to support that– parents pass on to their offspring discrete “heritable factors” and – the heritable factors (today called genes), retain their individuality generation after generation.
Removal ofstamens
Carpel
White
Stamens
Transferof pollenPurple
Carpel maturesinto pea pod
Seeds frompod planted
Offspring(F1)
Parents(P)
2
3
1
4
Experimental genetics began in an abbey garden
True-breeding varieties result when self-fertilization produces offspring all identical to the parent.
The offspring of two different varieties are hybrids.
The cross-fertilization is a hybridization, or genetic cross.
Parental plants are the P generation.
Offspring/Filial = F1 generation.
A cross of F1 plants produces an F2 generation.
Genetics Vocabulary
Gene-segment of DNA that had the coding for a particular trait.
Allele- one of several varieties of a gene.
Locus- location on a chromosome where the gene is located
Testcross-mating between an individual of unknown genotype and a known(homozygous recessive individual)
… dominant, recessive, homozygous, heterozygous, phenotype, genotype
The Experiment
P generation(true-breedingparents)
F1 generation
F2 generation
of plantshave purple flowers
of plantshave white flowers
Purpleflowers
Whiteflowers
All plants havepurple flowers
Fertilizationamong F1 plants(F1 F1)
34
14
Character TraitsDominant Recessive
Flower color
Purple White
Flower position
Axial Terminal
Seed colorYellow Green
Seed shapeRound Wrinkled
Pod shape
Inflated Constricted
Pod color
Green Yellow
Stem length
Tall Dwarf
The seven pea characteristicsstudied by Mendel
Mendel’s Discoveries
• Principle of Dominance
• Law (Principle) of Segregation
• Law (Principle) of Independent Assortment
Mendel’s Theory of Segregation
An individual inherits a unit of information (allele) about a trait from each parent
During gamete formation, the alleles segregate from each other (homologous chromosomes separate from each other, one allele of each gene from each parent is passed to offspring)
Alleles
• Different molecular forms of a gene
• Arise by mutation
Homologous chromosomes bear the alleles for each character
A locus (plural, loci) is the specific location of a gene along a chromosome.
For a pair of homologous chromosomes, alleles of a gene reside at the same locus.
The law of independent assortment is revealed by tracking two characters at once
A dihybrid cross is a mating of parental varieties that differ in two characters.
Mendel performed the following dihybrid cross with the following results:– P generation: round yellow seeds wrinkled green seeds
– F1 generation: all plants with round yellow seeds
– F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
41
Sperm
Eggs
Yellowround
Greenround
Yellowwrinkled
Greenwrinkled
The hypothesis of independent assortmentActual results; hypothesis supported
RY
RY
rY
rY
Ry
Ry ry
ry
RRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy RRyy Rryy
RrYy rrYy Rryy rryy
41
41
41
41
41
41
41
169
163
161
F1 generation RrYy
163
F1 generation
41
41
41
41
All yellow round seeds(RrYy)
Meta-phase I
of meiosis
Anaphase I
Metaphase II
Fertilization
Gametes
F2 generation 9 :3 :3 :1
RY ry rY Ry
R
R
R
yy
y
rr
r
YY
Y
YY
Y
R
R R
r
rr
y
yy
Y
Y
y
y
R
R
r
r r
r
r
R
R
R
Y
Y
Y
y
y
y
Allelic combinationspossible in gametes
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
Mendel concluded that the two “units” for a trait were to be assorted into gametes independently of any other “units” for the other traits
Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis
Mendel’s Law of Independent Assortment
Genetics is more complicated than Mendel’s experiments lead him to understand….
Dominance Relationships: – Complete dominance (Simple dominance)
– Incomplete dominance
– Codominance
Multiple AllelesX-linked
Gene Interactions: - Pleiotrophy - Polygenic
Inheritance Patterns
Incomplete Dominance
XHomozygous parent
Homozygous parent
All F1 are heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Incomplete Dominance- Example snapdragons
Another example of incomplete dominance in humans is hypercholesterolemia, dangerously high levels of cholesterol occur in the blood - heterozygotes have intermediately high cholesterol levels.
Blood Group(Phenotype) Genotypes
Carbohydrates Presenton Red Blood Cells
AntibodiesPresentin Blood
A
B
AB
O
IAIA
orIAi
IBIB
orIBi
IAIB
ii
Carbohydrate A
Carbohydrate B
Carbohydrate A
and
Carbohydrate B
Neither
Anti-B
Anti-A
Anti-B
Anti-A
None
No reaction Clumping reaction
O A B AB
Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left
Many genes have more than two alleles in the population (multiple alleles)
Human ABO blood group phenotypes involve three alleles for a single gene.
Chromosomes determine sex in many species
Many animals have a pair of sex chromosomes,• designated X and Y,
• that determine an individual’s sex.
In mammals,• males have XY sex chromosomes,
• females have XX sex chromosomes,
• the Y chromosome has the SRY gene for the development of testes, and
• an absence of the Y allows ovaries to develop.
Chromosomes determine sex in many species
Some organisms lack sex chromosomes altogether.
In bees, sex is determined by chromosome number.
• Females are diploid.
• Males are haploid.
In some species sex is determined by the temperature at which the eggs are incubated. (some crocodiles and turtles)
Sex-linked genes are located on either of the sex chromosomes.
The X chromosome carries many genes unrelated to sex. These genes are called X-linked.
Ex- The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.
Sex-linked genes exhibit a unique pattern of inheritance
Sex-linked traitsExamples- red-green colorblindness, Hemophilia, Duchenne’s muscular dystrophy
Cross a colorblind male with a female that is a carrier for the trait.
_______x _______
X- Inactiviation
Nondisjunction
A single character may be influenced by many genes
Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
A single gene may affect many phenotypic characters
Pleiotropy occurs when one gene influences many characteristics.
Sickle-cell disease is a human example of pleiotropy. This disease• affects the type of hemoglobin produced and the shape of red
blood cells and
• causes anemia and organ damage.
• Sickle-cell and nonsickle alleles are codominant.
• Carriers of sickle-cell disease are resistant to malaria.
Non-Nuclear Inheritance
The environment affects many characters
Many characters result from a combination of heredity and the environment. For example,• skin color is affected by exposure to sunlight,
• susceptibility to diseases, such as cancer, has hereditary and environmental components, and
• identical twins show some differences.
Only genetic influences are inherited. Nature v. Nuture ( Genes v. Environment)
Environmental Effects on Plant Phenotype
• Hydrangea macrophylla• Action of gene responsible for floral color is
influenced by soil acidity• Flower color ranges from pink to blue
Environmental Effects on Plant Phenotype (Yarrow, Achillea millefolium)
Temperature Effects on Phenotype
• Rabbit is homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway
• Melanin is produced in cooler areas of body
Genetic traits in humans can be tracked through family pedigrees
In a simple dominant-recessive inheritance ; one allele is dominant and the other is recessive
P= purple p= white
Cross a heterozygous purple flowering plant with a plant that had white flowers.
Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.
Dominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
Examples of single-gene inherited traits in humans
Genetic traits in humans can be tracked through family pedigrees
A pedigree
Many inherited disorders in humans are controlled by a single gene
The most common genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is
– Recessive
– Carried by about 1 in 31 Americans.
– Shortened life expectancy (40-50 years)
Dominant human disorders include
– Achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the nervous system.
New technologies offer ways to obtain genetic information
– before conception,
– during pregnancy, and
– after birth.
Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.
New technologies can provide insight into one’s genetic legacy
Several technologies can be used for detecting genetic conditions in a fetus.
– Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits
– karyotyping and
– biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease.
– Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests.
New technologies can provide insight into THE OFFSPRING’s genetic legacy
Amniocentesis
Ultrasoundtransducer
Fetus
Placenta
UterusCervix
Amniotic fluidextracted
Centrifugation
Amniotic fluid
Fetal cells
Culturedcells
Several hours
Several weeks
Several weeks
Biochemicaland geneticstests
Several hours
Several hours
Fetal cells
Cervix
Uterus
Chorionicvilli
PlacentaFetus
Ultrasoundtransducer
Tissue extractedfrom the chorionic villi
Chorionic Villus Sampling (CVS)
Karyotyping
Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.
Fetal imaging, the most common procedure is ultrasound imaging, uses sound waves to produce a picture of the fetus.
Newborn screening can detect diseases that can be prevented by special care and precautions.
New technologies can provide insight into THE OFFSPRING’s genetic legacy
New technologies raise ethical considerations that include
– the confidentiality and potential use of results of genetic testing,
– time and financial costs, and
– determining what, if anything, should be done as a result of the testing.
New technologies can provide insight into one’s genetic legacy
Genes on the same chromosome tend to be inherited together
Linked genes, which
– are located close together on the same chromosome and
– tend to be inherited together.
Crossing over produces new combinations of alleles
Crossing over between homologous chromosomes produces new combinations of alleles in gametes / new chromosomes!.
Geneticists use crossover data to map genes
When examining recombinant frequency (crossing over), the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.
Recombination frequencies can thus be used to map the relative position of genes on chromosomes.
•
Female MaleHemophilia
Carrier
NormalAlexis
Alexandra CzarNicholas IIof Russia
QueenVictoria
Alice Louis
Albert
Most compounds are synthesized by a sequence of metabolic steps involving many enzymes. If the enzymes (proteins) are not present or altered in some way (as in- the instructions coded for in the gene are altered) the compound will be affected.
In addition to simple gene inheritance; environmental factors, interactions between genes, mutations, even nutrition can have an effect on how a gene is expressed. This makes genetics a difficult and complex field of study.
1. Define and distinguish between these terms: the P generation, the F1 generation, and the F2 generation.
2. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square.
You should now be able to
3. Name and explain Mendel’s laws.
4. Describe the structure of homologous chromosomes.
5. Explain how family pedigrees can help determine the inheritance of many human traits.
6. Explain how recessive and dominant disorders are inherited. (be able to use a Punnett square)
7. Describe the types and use of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging.
You should now be able to
8. Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance. Be able to do crossed involving incomplete dominance and ABO blood group.
You should now be able to
10. Define the term: linked genes.
11. Explain how sex is genetically determined in humans and the significance of the SRY gene.
12. Describe patterns of sex-linked inheritance and examples of sex-linked disorders. Be able to do a Punnett Square.
You should now be able to