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7.1 Chromosomes and Phenotype

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1 7.1 Chromosomes and Phenotype
KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits.

2 Two copies of each autosomal gene affect phenotype.
An autosomal gene is a gene located on a numbered chromosome and usually affects males and females in the same way. Phenotype is the physical and psychological characteristics of an organism from both genetics and environment. Mendel studied autosomal gene traits like hair texture, widow’s peak, hitchhiker’s thumb, ear attachment, etc...

3 Mendel’s rules of inheritance apply to autosomal genetic disorders.
An organism's genotype represents the two alleles inherited for a given trait such as CC or cc. For an organism to be a carrier, the genotype must include one copy of a recessive allele (Ex. Bb). Carriers do not exhibit the physical trait, but have a 50% chance of passing the gene on to an offspring. Disorders caused by dominant alleles are uncommon. (dominant)

4 Males and females can differ in sex-linked traits.
Genes on sex chromosomes are called sex-linked genes. Y chromosome genes in mammals are responsible for male characteristics. X chromosome genes in mammals affect many traits.

5 Male mammals have an XY genotype.
All of a male’s sex-linked genes are expressed. Males have no second copies of sex-linked genes.

6 Female mammals have an XX genotype.
Expression of sex-linked genes is similar to autosomal genes in females. X chromosome inactivation randomly “turns off” one X chromosome.

7 7.2 Complex Patterns of Inheritance
KEY CONCEPT Phenotype is affected by many different factors.

8 Phenotype can depend on interactions of alleles.
In incomplete dominance, neither allele is completely dominant nor completely recessive. Heterozygous means that an organism has two different alleles of a gene (Hh or Rr). Homozygous means that the organism has two copies of the same allele for a gene (HH or hh).

9 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.

10 Many genes may interact to produce one trait.
Polygenic traits are produced by two or more genes (at least 3 genes and 6 alleles). Height in addition to hair, eye, and skin color are examples. Order of dominance: brown > green > blue.

11 An epistatic gene can interfere with other genes
An epistatic gene can interfere with other genes. It hides the output of a gene or genes. Examples are red hair and albinism.

12 The environment interacts with genotype.
Phenotype is a combination of genotype and environment. The sex of sea turtles depends on both genes and the environment; females develop with warmer temperatures (~88°F) whereas males develop in slightly cooler temperatures (~83°F). Height is an example of a phenotype strongly affected by factors in the environment such as nutrition and diseases.

13 7.3 Gene Linkage and Mapping
KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

14 Gene linkage was explained through fruit flies.
Nobel prize winner Thomas Hunt Morgan found that linked traits are on the same chromosome based on experiments with fruit flies. Chromosomes, not genes, assort independently or “crossover” during meiosis. Wild type Mutant

15 Linked genes are not inherited together every time.
Chromosomes exchange homologous genes (HH or hh) during meiosis.

16 Linkage maps estimate distances between genes.
The closer together two genes are, the more likely they will be inherited together. Cross-over frequencies are related to distances between genes. Linkage maps show the relative locations of genes.

17 Cross-over frequencies can be converted into map units.
gene A and gene B cross over 6.0 percent of the time gene B and gene C cross over 12.5 percent of the time gene A and gene C cross over 18.5 percent of the time

18 7.4 Human Genetics Pedigrees
KEY CONCEPT A combination of methods is used to study human genetics.

19 Human genetics follows the patterns seen in other organisms.
The basic principles of genetics are the same in all sexually reproducing organisms. Inheritance of many human traits is complex. Single-gene traits are important in understanding human genetics.

20 Females can carry sex-linked genetic disorders.
Males (XY) express all of their sex linked genes. Expression of the disorder depends on which parent carries the allele and the sex/gender of the child. X Y

21 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.

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

23 Several methods help map human chromosomes.
A karyotype is a picture of all chromosomes in a cell. X Y

24 Karyotypes can show changes in chromosomes.
deletion of part of a chromosome or loss of a chromosome large changes in chromosomes extra chromosomes or duplication of part of a chromosome

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