7.1 key concept the chromosomes on which genes are located can affect the expression of traits

Ch. 7: Extending Mendelian Genetics

7.1 KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits.


Two copies of each autosomal gene affect phenotype.

• Mendel studied autosomal gene traits, like hair texture.


• Mendel’s rules of inheritance apply to autosomal genetic disorders.

– Recessive disorder requires a homozygous recessive genotype to affect a person. A heterozygote for a recessive disorder is a carrier.

– Disorders caused by dominant alleles are uncommon.

(dominant)


• Mendel’s rule only apply for autosomal genes(chromosome pairs 1-22). Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for

male characteristics. Males have an XY genotype.– X chromosome genes in mammals affect many traits.

Females have an XX genotype.


– Because males only have one copy of the X chromosome all of a male’s sex-linked genes are expressed.

– In females, expression of sex-linked genes is similar to autosomal genes because they have two copies of the X chromosome.


– Common examples of sex-linked disorders:• Duchenne’s muscular dystrophy• Hemophilia• Color Blindness


Normal Color Vision Red-Green Color Blind

Left Right Left Right

Top 25 29 Top 25 Spots

Middle 45 56 Middle Spots 56

Bottom 6 8 Bottom Spots Spots

The test to the left is simpler. The individual with normal color vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.


– X chromosome inactivation randomly “turns off” one X chromosome.


• In incomplete dominance, neither allele is completely dominant nor completely recessive.


• Codominant alleles will both be completely expressed.


• Codominant alleles will both be completely expressed.

– Codominant alleles are neither dominant nor recessive.

– The ABO blood types result from codominant alleles.

• Many genes have more than two alleles.


• There are 3 alleles that determine blood type: IA, IB, and i– IA and IB are dominant to i.– IA and IB are codominant.


Codominance


– There are 4 blood types:A :

• Genotyple: IAIA or IAi• Antigen: A

• Antibody: Anti-B

• Can receive from: A, O

• Can donate to: A, AB


B :• Genotyple: IBIB or IBi• Antigen: B

• Antibody: Anti-A

• Can receive from: B, O

• Can donate to: B, AB


AB :• Genotyple: IAIB

• Antigen: A & B

• Antibody: none

• Can receive from: A, B, AB, & O (universal recipient)

• Can donate to: AB


O :• Genotyple: ii• Antigen: none

• Antibody: Anti-A and Anti-B

• Can receive from: O

• Can donate to: A, B, AB, & O (universal donor)


• Rh Factor: Another surface marker which may or may not be present on the RBC.


Rh+ :• Genotype: Rh+Rh+ or Rh+Rh-

• Antigen: Rh

• Antibody: none

• Can receive from: Rh+ & Rh-

• Can donate to: Rh+


Rh- :• Genotype: Rh-Rh-

• Antigen: none• Antibody: Rh+

• Can receive from: Rh-

• Can donate to: Rh- & Rh+


• Many genes have more than two alleles or multiple alleles used to express a trait.


• Polygenic traits are produced by two or more genes.

Order of dominance: brown > green > blue.


• An epistatic gene can interfere with other genes.


Epistatic: recessive trait blocks another alleleBlack is dominant to chocolate B or bYellow is recessive epistatic (E or e)

PhenotypePossible Genotypes

BBEE BbEE BBEe BbEe

bbEEbbEe

BBeeBbeebbee


Phenotype can be a combination of genotype and environment.

Malnourished Fed and cared for.

Identical twins separated at birth and treated differently.


7.4 KEY CONCEPT A combination of methods is used to study human genetics.


• A karyotype is a picture of all chromosomes in a cell.

X Y


• Karyotypes can show changes in chromosomes.

– Nondisjunction: an error in meiosis that results in an extra or a loss of a chromosome

– deletion of part of a chromosome or duplication of part of a chromosome


A pedigree is a chart for tracing genes in a family.

• Phenotypes are used to infer genotypes on a pedigree.• Autosomal genes show different patterns on a pedigree

than sex-linked genes.


• If the phenotype is more common in males, the gene is likely sex-linked.


7.3 KEY CONCEPT Genes can be mapped to specific locations on chromosomes.


• Because of their location on a chromosome, some genes travel together and are considered to be linked.

Wild type Mutant


Linkage maps estimate distances between genes.

• The closer together two genes are, the more likely they will be inherited together.

• Linkage maps show the relative locations of genes.

7.1 key concept the chromosomes on which genes are located can affect the expression of traits

Documents