BEYOND DOMINANT & RECESSIVE ALLELES

INCOMPLETE DOMINANCE, CODOMINANCE, MULTIPLE ALLELES, POLYGENIC TRAITS

DOMINANT? RECESSIVE? NEITHER?

NOT ALL GENES SHOW SIMPLE PATTERNS OF DOMINANT AND RECESSIVE ALLELES. IN MOST ORGANISMS, GENETICS IS MORE COMPLICATED, BECAUSE THE MAJORITY OF GENES HAVE MORE THAN TWO ALLELES. MANY IMPORTANT TRAITS ARE CONTROLLED BY MORE THAN ONE GENE.

INCOMPLETE DOMINANCEPAIRS OF ALLELES PRODUCE A HETEROZYGOUS PHENOTYPE THAT RESULTS IN AN APPEARANCE IN BETWEEN THE PHENOTYPES OF THE TWO PARENTAL VARIETIES.

EXAMPLE: SNAPDRAGON COLOR

INCOMPLETE DOMINANCEWith incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

INCOMPLETE DOMINANCE - SNAPDRAGONS

INCOMPLETE DOMINANCE – Humans

HYPERCHOLESTEROLEMIA – DANGEROUSLY HIGH LEVELS OF CHOLESTEROL IN THE BLOOD

NORMAL INDIVIDUALS ARE HHHETEROZYGOTES ARE Hh (1 IN 500)HOMOZYGOUS INDIVIDUALS WITH hh (1 IN

1,000,000) 5X NORMAL CHOLESTEROL LEVEL

INCOMPLETE DOMINANCE PROBLEMS

YELLOW GUINEA PIGS CROSSED WITH WHITE GUINEA PIGS ALWAYS PRODUCE CREAM COLORED GUINEA PIGS. A. COMPLETE THE PUNNETT SQUARE FOR THE

CROSS BETWEEN A WHITE GUINEA PIG AND A YELLOW GUINEA PIG.

B. COMPLETE THE PUNNETT SQUARE FOR THE CROSS BETWEEN TWO CREAM-COLORED GUINEA PIGS.

C. GIVE THE PHENOTYPIC AND GENOTYPIC RATIO FOR THE OFFSPRING IN A AND B.

CODOMINANCEBOTH ALLELES CONTRIBUTE TO THE

PHENOTYPE OF THE ORGANISMEXAMPLE: IN HORSES, THE ALLELE FOR

RED HAIR IS CODOMINANT WITH THE ALLELE FOR WHITE HAIR. HORSES WITH BOTH ALLELES ARE ROAN BECAUSE THEIR COATS ARE A MIXTURE OF BOTH RED AND WHITE HAIRS

CODOMINANCE

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 

CODOMINANCE PROBLEMS1. WHEN A RED BULL IS BRED TO A

WHITE COW, THE COLOR OF THE CALF IS ROAN. A. COMPLETE THE PUNNETT SQUARE FOR THE

CROSS BETWEEN A RED BULL AND A WHITE COW B. GIVE THE GENOTYPIC RATIO AND PHENOTYPIC

RATIO OF THE OFFSPRING. C. A ROAN BULL AND A ROAN COW ARE BRED.

GIVE THE GENOTYPIC RATIO AND PHENOTYPIC RATIO OF THIS CROSS.

MULTIPLE ALLELESMORE THAN TWO POSSIBLE ALLELES

EXIST IN A POPULATIONEX. COAT COLOR IN RABBITS –

DETERMINED BY A SINGLE GENE WITH AT LEAST 4 DIFFERENT ALLELES THAT CAN PRODUCE 4 POSSIBLE COAT COLORS

EX. HUMAN GENE FOR EYE COLOR

Multiple Alleles

Human Eye Color3 known genes that Explain typical patterns of green, brown,

blue eye color inheritanceHazel, grey and multiple shades of green,

brown, blue are not explained – molecular basis unknown

Human Eye Color Genes

Human Eye Color Genes:EYCL1 (gey) green dominant over blueGreen/blue eye color, EYCL2 (bey 1)EYCL3 (bey 2) brown dominant over blueCentral Brown eye color genehttp://www.athro.com/evo/gen/eyecols.html

MULTIPLE ALLELESHUMAN BLOOD TYPES – CHROMOSOME 9ONE GENE, THREE ALLELES, PRODUCE 4

PHENOTYPESA PERSON’S BLOOD GROUP MAY BE O, A,

B OR ABTHESE LETTERS REFER TO 2

CARBOHYDRATES THAT MAY BE FOUND ON THE SURFACE OF RED BLOOD CELLS (RBC)

ABO BLOOD GROUPS

Blood Types: What do they look like?

ABO BLOOD GROUPS

Blood Type GenotypesABO EnzymesPresent

RBC AntigensPresent

Serum Antibodies

"A" AA, Ai "H", "A" A, H anti-B

"B" BB, Bi "H", "B" B, H anti-A

"AB" AB "H", "A", "B" A, B, H none

"O" ii "H" H anti-A, anti-B

ABO BLOOD GROUP PROBLEMS

COMPLETE A PUNNETT SQUARE FOR:1. HOMOZYGOUS TYPE A x TYPE O2. HETEROZYGOUS TYPE A x TYPE O3. HOMOZYGOUS TYPE A x HOMOZYGOUS TYPE B4. HETEROZYGOUS TYPE B x HOMOZYGOUS TYPE A5. HETEROZYGOUS TYPE A x HETEROZYGOUS TYPE B6. HETROZYGOUS TYPE B x TYPE O

Practice problemsA TYPE AB MALE MARRIES A TYPE O

FEMALE.

WHAT ARE THEIR GENOTYPES?

COMPLETE A PUNNETT SQUARE TO SHOW THE POSSIBLE GENOTYPES OF THEIR CHILDREN.

Practice problemsA TYPE A MALE MARRIES A TYPE B

FEMALE. THEY HAVE TWO CHILDREN. ONE CHILD IS TYPE AB AND THE OTHER CHILD IS TYPE O.

WHAT ARE THE GENOTYPES OF THE PARENTS?

COMPLETE A PUNNETT SQUARE TO SHOW THE GENOTYPES OF THESE TWO CHILDREN AND FOR ANY FUTURE CHILDREN.

Practice problemsA TYPE A MALE IS MARIED TO A TYPE O

FEMALE. USING PUNNETT SQUARES, EXPLAIN HOW IT WOULD BE POSSIBLE FOR THEM TO HAVE A:

TYPE A CHILD.

TYPE B CHILD.

TYPE O CHILD.

Practice problemsA TYPE AB MALE MARRIES A TYPE B

FEMALE. USING PUNNETT SQUARES, EXPLAIN HOW IT WOULD BE POSSIBLE FOR THEM TO HAVE A:

TYPE AB CHILD

TYPE A CHILD

TYPE O CHILD

TYPE B CHILD

Practice problemsA TYPE AB MALE MARRIES A TYPE AB

FEMALE. USING PUNNETT SQUARES, EXPLAIN HOW IT WOULD BE POSSIBLE FOR THEM TO HAVE A:

TYPE AB CHILD

TYPE A CHILD

TYPE B CHILD

TYPE O CHILD

Practice problemsA TYPE O MALE MARRIES A TYPE O

FEMALE.

WHAT ARE THE POSSIBLE GENOTYPES OF THEIR CHILDREN?

Practice problemsA TYPE A MALE, WHOSE FATHER IS TYPE O,

MARRIES A TYPE O FEMALE. WHAT ARE THE POSSIBLE GENOTYPES OF THEIR CHILDREN?

THE MALE FROM THE PREVIOUS PROBLEM HAS A BROTHER WHO IS TYPE B. WHAT IS THEIR MOTHER’S BLOOD TYPE?

THE BROTHERS IN THE ABOVE PROBLEMS HAVE A SISTER WHOSE BLOOD TYPE IS O. IS THIS POSSIBLE?

POLYGENIC TRAITSTRAITS CONTROLLED BY TWO OR MORE

GENESEX. 3 GENES INVOLVED MAKING THE

REDDISH-BROWN PIGMENT IN THE EYES OF FRUIT FLIES

EX. WIDE RANGE OF HUMAN SKIN COLOR DUE TO MORE THAN 4 DIFFERENT GENES CONTROLLING THE TRAIT

POLYGENIC TRAIT – COAT COLOR IN MICE

SEX-LINKED TRAITSIN HUMANS, SEX IS DETERMINED BY

THE 23RD PAIR OF CHROMOSOMES – THE SEX CHROMOSOMES!

THE SEX CHROMOSOMES ARE THE X AND THE Y CHROMOSOMES XX – FEMALE XY – MALE

SEX-LINKED TRAITSMOST SEX-LINKED GENES ARE X-

LINKED GENES.WHY? THE X CHROMOSOMES ARE

LONGER AND CONTAINS THOUSANDS MORE GENES THAN THE Y CHROMOSOME.

X CHROMOSOME CONTAINS 1098 GENES.Y CHROMOSOME CONTAINS 26 GENES.

KARYOTYPE

SEX-LINKED TRAITS

FOR EACH GENE EXCLUSIVELY ON THE X CHROMOSOME, THERE ARE TWO ALLELES OF EACH GENE

MALES, XY, HAVE ONLY ONE ALLELEA MALE, XY, WITH A RECESSIVE ALLELE ON

THE X CHROMOSOME, WILL ALWAYS EXHIBIT THAT RECESSIVE TRAIT BECAUSE THERE IS NO CORRESPONDING ALLELE ON THE Y CHROMOSOME.

EXAMPLES OF SEX-LINKED TRAITS

IN HUMANS: Red-Green Color Blindness Hemophilia Duchenne Muscular Dystrophy

IN CATS Calico coat color

IN FRUIT FLIES: Eye Color

WORKING PUNNETT SQUARES

IDENTIFY EACH INDIVIDUAL AS MALE OR FEMALE ACCORDING TO THEIR SEX CHROMOSOMES EX. XX or XY

THE LETTERS REPRESENTING THE TRAIT ARE PLACED AS SUPERSCRIPTS ABOVE THE X CHROMOSOME EXAMPLE: XR or Xr

EYE COLOR IN FRUIT FLIESTHE GENE FOR EYE COLOR IS ON THE X CHROMOSOME.

THE ALLELE FOR RED EYES IS DOMINANT OVER WHITE EYES

PROBLEM: IF A WHITE-EYED FEMALE FRUIT FLY IS MATED WITH

A RED-EYED MALE, PREDICT THE POSSIBLE OFFSPRING

COAT COLOR IN CATSCoat color in cats is an X-linked gene, with

alleles for black and orange-brown XBXB and XBY cats will have a black coat XOXO and XOY will have an orange-brown coat female cats with XBXO are Calico!!

Pedigrees

a diagram of a family history that shows relationships between family members and their status with respect

to a particular hereditary condition. The affected person through which the pedigree is

discovered is indicated by an arrow on pedigrees. Males are denoted by squares and females by circles.

A filled symbol indicates that an individual is affected by a specific condition.

Generations are represented on horizontal levels; parents on one level and the children of those parents

together on a level below them.

Pedigrees Autosomal Dominant

Autosomal DominantDominant conditions are expressed in individuals

who have just one copy of the mutant allele. The pedigree on the right illustrates the transmission of an autosomal dominant trait. Affected males and females have an equal probability of passing on the trait to offspring. Affected individual's have one normal copy of the gene and one mutant copy of the gene, thus each offspring has a 50% chance on inheriting the mutant allele. As shown in this pedigree, approximately half of the children of affected parents inherit the condition and half do not.

Pedigrees–Autosomal Recessive

Autosomal RecessiveRecessive conditions are clinically manifest only

when an individual has two copies of the mutant allele. When just one copy of the mutant allele is present, an individual is a carrier of the mutation, but does not develop the condition. Females and males are affected equally by traits transmitted by autosomal recessive inheritance. When two carriers mate, each child has a 25% chance of being homozygous wild-type (unaffected); a 25% chance of being homozygous mutant (affected); or a 50% chance of being heterozygous (unaffected carrier).

Pedigrees-X-Linked Recessive

X-Linked Recessive X-linked recessive traits are not clinically manifest when

there is a normal copy of the gene. All X-linked recessive traits are fully evident in males because they only have one copy of the X chromosome, thus do not have a normal copy of the gene to compensate for the mutant copy. For that same reason, women are rarely affected by X-linked recessive diseases, however they are affected when they have two copies of the mutant allele. Because the gene is on the X chromosome there is no father to son transmission, but there is father to daughter and mother to daughter and son transmission. If a man is affected with an X-linked recessive condition, all his daughter will inherit one copy of the mutant allele from him.

Pedigrees-X-Linked Dominant

X-Linked Dominant

Because the gene is located on the X chromosome, there is no transmission from father to son, but there can be transmission from father to daughter (all daughters of an affected male will be affected since the father has only one X chromosome to transmit). Children of an affected woman have a 50% chance of inheriting the X chromosome with the mutant allele. X-linked dominant disorders are clinically manifest when only one copy of the mutant allele is present.

Famous Pedigree

The PEDIGREE ofNicholas (Nikolia) II ROMANOV (last CZAR)

of RUSSIA

(Nicholas Alexandrovitch); Knight of the Garter; (since he possessed Russia, he may be considered the wealthiest man ever)