HEREDITY: Going beyond HEREDITY: Going beyond MendelMendel

Individuals don’t always Individuals don’t always follow the basic pattern of follow the basic pattern of

inheritance inheritance (dominant/recessive)(dominant/recessive)

Many inheritance patterns are Many inheritance patterns are more complex:more complex:

• Linked genesLinked genes

• Incomplete/codominanceIncomplete/codominance

• Multiple allelesMultiple alleles

• Sex Determination/Sex-linked traitsSex Determination/Sex-linked traits

• Polygenic InheritancePolygenic Inheritance

• Effect of environment on gene Effect of environment on gene expressionexpression

Independent AssortmentIndependent Assortment

• Recall……..Genes on separate Recall……..Genes on separate chromosomes will be inherited chromosomes will be inherited separately.separately.

• The way in which chromosomes line The way in which chromosomes line up during meiosis is randomup during meiosis is random

• This adds to variationThis adds to variation

LINKED GENESLINKED GENES

• Chromosomes carry MANY genes, so Chromosomes carry MANY genes, so some genes will be on the same some genes will be on the same chromosome.chromosome.

• Genes on the same chromosome are Genes on the same chromosome are called LINKED genes and are usually called LINKED genes and are usually inherited together.inherited together.

• I say usually because what can I say usually because what can happen during Prophase I of meiosis?happen during Prophase I of meiosis?

Incomplete dominanceIncomplete dominance

• Neither allele completely masks the Neither allele completely masks the expression of the otherexpression of the other

• Heterozygous individuals have a Heterozygous individuals have a different phenotype than either different phenotype than either parentparent

• There is an intermediate phenotype, There is an intermediate phenotype, sometimes called blended sometimes called blended inheritanceinheritance

Example of incomplete Example of incomplete dominance:dominance:

• The snapdragonThe snapdragon– The red snapdragon is homozygous and The red snapdragon is homozygous and

the white snapdragon is also the white snapdragon is also homozygous. The red does not homozygous. The red does not completely mask the white, so a pink completely mask the white, so a pink phenotype is the result of a cross phenotype is the result of a cross between red and white. For this cross, between red and white. For this cross, we can use different letters:we can use different letters:

– RR x WW = RWRR x WW = RW

• In humans, spacing of eyes and wavy In humans, spacing of eyes and wavy hair are examples of incomplete hair are examples of incomplete dominancedominance

Co-dominanceCo-dominance

• Similar to the concept of incomplete Similar to the concept of incomplete dominance. No allele masks the dominance. No allele masks the otherother

• In co-dominance, BOTH traits show in In co-dominance, BOTH traits show in the phenotypethe phenotype

• Example-barred rock chicken is a Example-barred rock chicken is a result of a cross between a black result of a cross between a black chicken and a white chickenchicken and a white chicken

Example of co-dominance:Example of co-dominance:

• In humans, sickle cell anemiaIn humans, sickle cell anemia

• as well as types A and B blood show as well as types A and B blood show co-dominanceco-dominance

Multiple AllelesMultiple Alleles

• In some cases, an offspring inherits In some cases, an offspring inherits two alleles from the parent from a two alleles from the parent from a choice of three or more alleleschoice of three or more alleles

• Example is pigeon feather colorExample is pigeon feather color

Red is dominant to other alleles

Blue is recessive to red, dominant to choclate

Chocolate is recessive to both

Red can AA, Ab or Ac

Blue can be bb or bc

Chocolate will be cc

Examples of multiple Examples of multiple alleles:alleles:

• In humans, A, B, o blood types (not In humans, A, B, o blood types (not the positive/negative factors)the positive/negative factors)

Cross a father heterozygous for Cross a father heterozygous for B blood with a mother B blood with a mother heterozygous for A blood:heterozygous for A blood:• IIB B ii x I x IA A ii BB ii

AA ABAB AAii

ii BBii iiii

Type Type O O

bloodblood

Polygenic InheritancePolygenic Inheritance

• Inheritance pattern controlled by Inheritance pattern controlled by many genes (do not confuse this with many genes (do not confuse this with multiple alleles)multiple alleles)

• A trait controlled by polygenic A trait controlled by polygenic inheritance could have MANY inheritance could have MANY phenotypesphenotypes

• Results will show a bell curve or a Results will show a bell curve or a continuumcontinuum

Example of polygenic Example of polygenic inheritance:inheritance:

• Height and skin color in humansHeight and skin color in humans

Sex-linked InheritanceSex-linked Inheritance

• Sex-linked traits are controlled by Sex-linked traits are controlled by genes found on a sex chromosomegenes found on a sex chromosome

• The traits we study are found on the The traits we study are found on the X chromosome. The X chromosome X chromosome. The X chromosome carries different genes than the Y carries different genes than the Y chromosome carries.chromosome carries.

Sex-linked traits in humansSex-linked traits in humans

• Include red-green color-blindness and Include red-green color-blindness and hemophiliahemophilia

• These disorders are found more often These disorders are found more often in males because they just need one in males because they just need one copy of the defective allele to have copy of the defective allele to have the disorder.the disorder.

• Males do not have another copy of the Males do not have another copy of the genes on the X chromosome because genes on the X chromosome because they have only one X chromosome.they have only one X chromosome.

A color-blind son receives the A color-blind son receives the defective gene from his defective gene from his mother:mother:

Show a color blind father and a Show a color blind father and a carrier mother pass on their carrier mother pass on their genes:genes:

GENE EXPRESSIONGENE EXPRESSION

• Gene ExpressionGene Expression relates to the presence relates to the presence or dimension of the appearance of a or dimension of the appearance of a trait for which the genes are present.trait for which the genes are present.

• Internal environment (hormones, Internal environment (hormones, nutrition) and external environment nutrition) and external environment (temperature) may have an effect on (temperature) may have an effect on whether a gene is expressed and if so, whether a gene is expressed and if so, how much it is expressed.how much it is expressed.

Examples of gene Examples of gene expression:expression:

• Horn size in Bighorn sheepHorn size in Bighorn sheep

Genotypes are the same, but phenotypes are different, due to ?

Gene expression can also Gene expression can also be-be-

• Affected by temperature, as in a Affected by temperature, as in a weaselweasel

He is called an ermine in the winterHe is called a stoat in the

summer