Patterns of Inheritance

Chapter 9

An Old Genetic Experiment

• Genetics is the study of heredity/inheritance

• Dogs bred for specific traits

• Genome completed in 2003

• Wolves and domestic dogs share a common ancestor

History of Inheritance

• Blending hypothesis– Information (cells) from each parent produce mixed

offspring• Tall and short adults had medium height children

– Didn’t explain disappearance/reappearance of traits b/w generations

• Gregor Mendel 1860– Father of genetics– Parents pass on specific heritable factors to offspring

• ‘Genes’ don’t blend, but remain the same over generations

UsefulGenetics Terminology

Alleles

• Alternate versions of the same gene, located at the same loci on a specific chromosome– Dominant alleles mask others when both are present

(UPPERCASE LETTERS)• Dominance implies it determines phenotype, not superiority or

increased prevalence– Recessive alleles are easily masked by others (lowercase

letters)• Recessive traits often more common

• Individuals inherit 2 alleles 1 maternal and 1 paternal– Actual combination determines genotype– Resulting physical expression determines phenotype

Phenotype and Genotype• Homozygous dominant

(PP)– 2 dominant alleles– Express dominant trait

• Heterozygous (Pp)– 1 dominant and 1

recessive allele– Express dominant trait

• Homozygous recessive (pp)– 2 recessive alleles– Express recessive trait

• Phenotypic vs. genotypic ratios

Why a Pea?• Peas (Pisum sativum) have several

characters that vary among individuals

• Have distinct traits, or variants, of each character

• Can control types of mating/crosses that occurred– Self-fertilize natural, involves 1

plant– Cross-fertilize artificial, involves 2+

different plants• Can create true-breeding lines

– All individuals genetically identical– Only contain 1 character variant

CharactersTraits

Mendel’s Initial Work

• Started with monohybrid crosses– Differ by only 1 trait– Can use Punnett squares to represent hypothetical crosses

• All crosses produced same results– Crossing true-breeding tall and short (P) = only tall (F1)– Cross any resulting tall hybrids (F1) = 3:1 ratio (type of

ratio?) of tall to short (F2)– Short phenotype disappears but reappears in next

generation• Held true for all 7 tested characters

Constructing Punnett Squares• Allows determination of all possible genotypes

and phenotypes • Remember:

– All individuals have 2 alleles for every gene• 1 from mom and 1 from dad

– Meiosis produces haploid gametes from diploid cells• Aa mother = A or a eggs

• Steps– Place gametes (haploid) of one parent along top,

other along the left side– Combine all possible female gametes with all

possible male gametes = fertilization– Boxes with 2 alleles = possible offspring (diploids)

Practice

• Background:– Tall (T) and short (t)

• Fill in the boxes to show genotypes

• For each box, identify the phenotype

• For each punnett square list the genotypic and phenotypic ratios

Adopted from: http://www.exploringnature.org/db/detail.php?dbID=22&detID=2290

1._________ 2 ._________3._________ 4 ._________

1._________ 2 ._________3._________ 4 ._________

1._________ 2 ._________3._________ 4 ._________

1._________ 2 ._________3._________ 4 ._________

Identifying Individuals in Crosses

• P (parental) generation

• F1 (first filial) generation

• F2 (second filial) generation

• And so on …

Mendel’s Work (cont.)

• Mendel wanted to be able to identify genotypes of all individuals

• Designed a testcross– Cross recessive phenotype

with a dominant phenotype• Why is recessive phenotype

required?

– Determine genotype of a dominant trait

– Still used in current research

Mendel’s Work (cont.)

• Continued with dihybrid crosses– Peas differed by 2 characters

• All crosses produced same results– Crossing true-breeding round yellow and wrinkled

green (P) = only round yellow (F1)– Cross any resulting round yellow hybrids (F1) = 9:3:3:1

ratio (type of ratio?) of round yellow to round green to wrinkled yellow to wrinkled green(F2)

– Wrinkled green phenotype disappears but reappears in next generation along with 2 new phenotypes

Mendel’s Law of Segregation

• Two alleles of a trait separate during gamete formation

• Remember– Homologous chromosomes

each carry 1 allele– Meiosis separates these

chromosomes forms haploid gametes

Mendel’s Law of Independent Assortment• Genes located on different chromosomes are

inherited independently• E.g. hair color doesn’t determine eye color

Autosomal Recessive Disorders• Only affects homozygous recessive individuals

– Heterozygous is “carrier”– Prevents complete removal of allele from a

population• Albinism

– Lack of normal amounts of melanin (pigment) in body

• Cystic fibrosis– Thick mucus in lungs & digestive tract

• Cl- channel abnormality– Most common lethal genetic disorder among

Caucasians

Autosomal Dominant Disorders

• Affects all dominant phenotypes• Lethal types less common• Achondroplasia

– Embryonic cartilage in skeleton doesn’t develop properly

– “Dwarf”, average 4’ tall• Huntington’s Disease

– Nervous system deteriorates– Symptoms often not seen until after 30– Die in 40s or 50s

Pedigrees: an application practice problem

Recessive trait: attached earlobe

femaleaffectedunaffected

male

carrier

• Diagram family relationships and phenotypes

• Allow human heredity to be studied– Can’t control human mating, so look at

those naturally occurring– Can indicate type of gene responsible

• Sex-linked or autosomal recessive/dominant

• Can deduce genotypes of most members from phenotypes– Mendelian genetics and logic

Not All Genetics Are Simple• Mendel used characters exhibiting complete

dominance, offspring look like one of the two parents (simple)– Not applicable to all characters– Genotype and phenotype relationship not so simple

• Single genes can have alleles that aren’t completely dominant or recessive

• Characters can have 1+ genes (complex)– Basic principles of segregation and independent

assortment still apply

Variations on Mendelian Genetics

• Incomplete dominance• Codominance• Epistasis• Polygenic inheritance• Ignore environmental influences

– Nutrition can effect height, sun exposure can alter skin color, exercise can change build, etc.

– Nature vs nurture still major debate

Incomplete Dominance

• Heterozygote offspring has an intermediate of parent’s phenotype– Doesn’t support blending– Each genotype has own phenotype

• True breeding red and white cross– Homozygous red and white offspring– Heterozygous pink offspring– 1:2:1 is and genotypic and phenotypic

ratio

• Heterozygote offspring expresses two alleles at the same time

• Blood type– 3 alleles– 4 phenotypes– 6 genotypes

• Universal donor?• Universal acceptor?

Codominance

Epistasis• Gene at one locus alters the

phenotypic expression of another gene at a second locus

• Coats of mice and Labrador retrievers – B (black) & b (brown)– C (melanin) & c (no melanin)

• Possibilities– B_C_ = black– bbC_ = brown– B_cc and bbcc = white or yellow

Polygenic Inheritance

• An additive effect of 2+ genes on one phenotypic trait

• Range of small differences in a trait• Skin color due to different amounts

and types of melanin• Height, weight, and iris (eye) color

too

CHROMOSOMAL INHERITANCE

Human Sex Determination

• 2 sex chromosomes and 44 autosomes

• XX = female and XY = male– Eggs = all X and sperm = X or Y– Sperm cell determines sex

• Gene on Y chromosome responsible for ‘maleness’– SRY gene (TDF production) triggers testes

development – Without, ovaries develop

• Default sex is female

Other Sex Determining Systems

• Insects have 1 sex (X) chromosome– Females XX, males X0

• Bees and ants are haploid or diploid– Queen decides– Diploid females, haploid males

• Marine fish commonly change– Social hierarchy and balance of sex’s

• Alligators and turtles rely on incubation temperature

• Plants are complex

Sex-linked GenesGenes that reside on sex chromosomes, but unrelated to genetic sex

X chromosome in humans (generally)• Fathers pass X to all daughters, but no sons• Mothers pass X to all offspring

– Can you justify these statements?X-linked disorders more common and most likely to affect males

X chromosome is largerMale affected with 1 = hemizygousFemales affected with 2; 1 = carrierRepresented in crosses differently

Need sex chromosome and UPPER or lower case letter to imply affected or not (XnY = affected male, XNXn = carrier female)

• Y-linked is rare– Used to track ancestry through male lines

Sex-linked Genes• Genes that reside on sex chromosomes but unrelated to genetic sex

– X chromsome in humans (generally)– Fathers pass X to all daughters, but no sons– Mothers pass X to all offspring

• Can you justify these statements?• X-linked disorders more common and most likely to affect males

– X chromosome is larger– Males affected with 1 = hemizygous– Females affected with 2; 1 = carrier– Represented in crosses differently

• Need sex chromosome and UPPER or lower case letter to imply affected or not (XnY = affected male, XNXn = carrier female)

• Y-linked is rare– Used to track ancestry through male lines

Red-Green Color Blindness

• Involves several X-linked genes– Some heterozygous females affected (rare)

• N represents color-blind gene carried at an X chromosome loci

• XN = trait not present, Xn = trait present

F: normal; M: affected F: carrier; M: carrierF: carrier; M: normal

Hemophilia• X-linked recessive• Allele for clotting

factor VIII mutated• Introduced into ruling

houses of Russia and Europe