Patterns of Inheritance

Chapter 10

Genetics developed from curiosity about inheritance

• Gregor Mendel– Late 1800s– Pea plants

Genetics developed from curiosity about inheritance

• Trait= a variation of a particular character• Genetics= study of heredity• Self-fertilization= fertilization of an ovum

of a plant by a male gamete from the same flower – True breeding

• Cross-fertilization= sperm from the pollen of one flower fertilizes the eggs in the flower of a different plant

Mendel discovered that inheritance follows rules of chance

• Hybrids= the offspring of two different true-breeding varieties– P generation= P for parent

– F1 generation= F for filial (Latin for son)

– F2 generation= offspring of the F1 generation…“grandchildren”

• Monohybrid cross= a pairing in which the parent plants differ in only one (mono) character– Mendel crossed purple-flowered pea plants with

white-flowered pea plants

Mendel discovered that inheritance follows rules of chance

Mendel discovered that inheritance follows rules of chance

• Mendel not only used flower color, he also used flower position, seed color, seed shape, pod shape, pod color, and stem length

• He used monohybrid crosses to study the heredity of these traits

Mendel discovered that inheritance follows rules of chance

• Mendel’s 4 hypotheses:1. There are other forms of genes.

– Such as the gene for flower color– One form for purple flowers– Another for white flowers

– Alternative forms of genes are called alleles

2. For each inherited trait, an organism has two alleles for the gene controlling that trait…one from mom and one from dad.– Heterozygous= two alleles for a trait are

different– Homozygous= two alleles for a trait are the

same

Mendel discovered that inheritance follows rules of chance

3. When only one of the two different alleles appears to affect the trait, that allele is called the dominant allele. The other allele that does not appear is called the recessive allele.

– Flower color- Purple– P = dominant

– p= recessive

4. The two alleles for a trait separate during the formation of gametes (remember…meiosis!). Each gamete has only one allele for each trait. The union of sex cells, during fertilization, reunites the alleles to form pairs again.

Mendel discovered that inheritance follows rules of chance

• You can calculate the probabilities for different combinations of alleles resulting from a genetic cross

• Punnett square= a diagram that shows all possible outcomes of a genetic cross

Mrs. B’s QUICK QUIZ1. The father of genetics is

Friar TuckGregor Mendel

Anton van LeeuwenhoekBill Nye

Gregor Mendel

2. _____________ is the fertilization of an ovum of a plant by a male gamete from the same flower.

True breedingCross fertilizationSelf fertilization

Genetics

Self fertilization

3. A hybrids is the offspring of one true breeding plant.

TrueFalse

FalseHybrids= the offspring of two different true-

breeding varieties

Mendel discovered that inheritance follows rules of chance

• The genetic makeup , or combination of alleles is called the genotype– The genotypic ratio is 1PP:2Pp:1pp

• The observable trait is called the phenotype (purple flowers)– The 3:1 ratio is called a phenotypic ratio

Mendel discovered that inheritance follows rules of chance

• Testcross= breeds an individual of unknown genotype, but dominant phenotype with a homozygous recessive individual

• The appearance of the phenotype of the offspring will reveal the mystery plant

Mendel discovered that inheritance follows rules of chance

• Dihybrid cross= crossing individuals that differ in two characters

• Mendel used this when studying peas with– different colors (yellow =dominant) – different shapes (round= dominant)– RRYY– RrYy– Rryy– A punnett square can be used in the same way

Mendel discovered that inheritance follows rules of chance

• Mendel used dihydrid crosses to study all seven characteristics of pea plants

• Proposed his principle of independent assortment, which states that during gamete formation in an F2 cross, a particular allele for one character can pair up with a particular allele of another character– R can end up with Y or y– r can end up with Y or y– The alleles are sorted into gametes independently of

one another

There are many variations of inheritance patterns

• For some characters of organisms, neither allele is dominant

• The heterozygotes have a phenotype that is intermediate between the phenotypes to the two parents

• This is called incomplete dominance– Ex: Andalusian chickens– A heterozygote chicken=CBCW

– This is NOT blending, because the parent phenotypes can reappear in the F2 generation

There are many variations of inheritance patterns

• Many genes have multiple alleles, instead of just two– Ex: human blood type– A, B, AB, O– IA and IB display codominance, meaning that

a heterozygote expresses both traits– This is NOT intermediate, due to the fact that

the phenotype shows the separate traits of both alleles

There are many variations of inheritance patterns

• When multiple genes affect a character, the variation in phenotypes can increase

• When two or more genes affect a single character, it is called polygenic inheritance– ex: height in humans– 3 tall alleles: A, B, C– 3 short alleles: X, Y, Z– Intermediate inheritance– AABBCC= very tall– AZBBCC= slightly shorter, etc.

• Some characters are affected by dozens of genes, which lead to numerous combinations of alleles

There are many variations of inheritance patterns

• An individual’s phenotype depends on environment as well as on genes– Ex: tree– Temperature

• The product of a genotype is generally a range of possibilities influenced by the environment

Meiosis explains Mendel’s principles

• Chromosome theory of inheritance: genes are located on chromosomes, and the behavior of chromosomes during meiosis and fertilization accounts for inheritance

Meiosis explains Mendel’s principles

• Gene locus• When genes are located on separate

chromosomes, they sort independently of each other during meiosis

• Crossing over allows genes on the same chromosome to sort independently

• The tendency for alleles on one chromosome to be inherited together is called genetic linkage…the closer the 2 genes are on a chromosome, the greater the genetic linkage…the farther apart they are, the more likely a cross over will occur

Sex-linked traits have unique inheritance patterns

• Eggs contain what kind of chromosome?

X• Sperm contain what kind of chromosome?

Half have X and half have Y• An offspring’s sex is determined by whether or

not the sperm cell carries an X or a Y• Any gene that is located on a sex chromosome

is called a sex-linked gene• In humans, most are on the X chromosome

Sex-linked traits have unique inheritance patterns

• Thomas Morgan– Fruit flies

• Sex-linked disorders– Red-green colorblindness

• X-linked recessive• Recessive sex-linked traits are

much more common in men than

in women

Pedigrees

• Family tree that records and traces the occurrence of a trait in a family

• Circles = females• Squares = males• Horizontal lines

connecting a square to a circle means the couple had children

Pedigrees

• If shape has two colors the person in a carrier