Section 1 Chromosomes and Inheritance Chapter 12 Objectives • Distinguishbetween sex chromosomes and autosomes. • Explainthe role of sex chromosomes in sex determination. • Describehow an X- or Y-linked gene affects the inheritance of traits. • Explainthe effect of crossing-over on the inheritance of genes in linkage groups. • Distinguishbetween chromosome mutations and gene mutations.

Chapter 12 Relationship between genes and chromosomes • Genes are a segment of DNA that control a trait • Genes are found on chromosomes.

Chapter 12 • Sex Chromosomes and Autosomes • Sex chromosomes contain genes that determine an organism’s sex (gender). • The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes.

How sex is determined

Section 1 Chromosomes and Inheritance Chapter 12 Karyotypes: Male and Female

Chapter 12 How sex is determined • In mammals, an individual carrying two X chromosomes is female. • An individual carrying an X and a Y chromosome is male.

Section 1 Chromosomes and Inheritance Chapter 12 Effects of Gene Location • Sex-Linked Genes and Traits • Genes found on the X chromosome are X-linked genes. • A sex-linked trait is a trait whose allele is located on a sex chromosome. • Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait.

X-Linked traits • Found on the X chromosome • Show more in males because the unaffected X chromosome acts as dominant trait. In females the X’ acts as a recessive gene. • ♀=X’X carrier but does not have the trait • ♂=X’Y has the trait • Examples: 1. colorblindness is found in 8% of males. Click here to go to a web site that tests for colorblindness.

X-Linked traits 2. hemophilia: almost exclusively in males (blood cannot clot properly) 3. Duchenne muscular dystrophy (weakens and destroys muscle tissue; scoliosis, enlarged calves)

Section 1 Chromosomes and Inheritance Chapter 12 Effects of Gene Location, continued • Linked Genes • Pairs of genes that tend to be inherited together are calledlinked genes.

Linkage Groups • Each chromosome carries many genes. Two or more genes located on one chromosome are said to be linked. • The linked genes located on one chromosome form a linkage group. • Because they are on the same chromosome, they tend to be inherited together. • Crossing over allows for the exchanging of locations of genes among the chromosomes that carry them. • This process allows for new combinations of alleles that are not found in the parental gametes. (see fig 12-4)

Chapter 12 Chromosome Mapping • The farther apart two genes are located on a chromosome, the more likely a cross-over will occur. • Researchers use recombinant percentages to construct chromosome maps showing relative gene positions.

Chromosome Map • A chromosome map is a diagram that show where genes are located on a chromosome. • This is useful to scientists who want to determine where genes are located on a particular gene. • Chromosome 1 contains over 3000 genes and over 240 million base pairs, of which ~90% have been determined. (click here)

Section 1 Chromosomes and Inheritance Chapter 12 Mutations • Germ-cell mutationsoccur in gametes and can be passed on to offspring. • Somatic-cell mutationsoccur in body cells and affect only the individual organism.

Chapter 12 Mutations, continued • Chromosome Mutations • Chromosome mutationsare changes in the structure of a chromosome or the loss or gain of an entire chromosome.

Section 1 Chromosomes and Inheritance Chapter 12 Chromosomal Mutations

Chapter 12 Mutations, continued • Gene Mutations • Gene mutationsare changes in one or more of the nucleotides in a gene.

Section 1 Chromosomes and Inheritance Chapter 12 Gene Mutations

Section 2 Human Genetics Chapter 12 Objectives • Analyzepedigrees to determine how genetic traits and genetic disorders are inherited • Summarizethe different patterns of inheritance seen in genetic traits and genetic disorders. • Explainthe inheritance of ABO blood groups. • Comparesex-linked traits with sex-influenced traits. • Explainhow geneticists can detect and treat genetic disorders.

Section 2 Human Genetics Chapter 12 Inheritance of Traits • Pedigrees • Geneticists use pedigrees to trace diseases or traits through families. • Pedigrees are diagrams that reveal inheritance patterns of genes.

Pedigree for Cystic Fibrosis

Chapter 12 Genetic Traits and Disorders • Polygenic Inheritance • Polygenic- characters, such as skin color, are controlled by two or more genes. • Complex Characters • Complex characters, such as polygenic traits, are influenced by both genes and environment. • Multiple Alleles • Multiple-allele characters, such as ABO blood groups, are controlled by three or more alleles of a gene.

Multiple allele traits • Controlled by 3 or more alleles of the same gene that code for a single trait • Example: ABO blood types, controlled by the alleles IA,IB, and I • You only inherit two! **notice A and B are codominant (both are expressed)

Section 2 Human Genetics Chapter 12 Genetic Traits and Disorders, continued • X-Linked Traits • The gene forcolorblindness, an X-linked recessive gene, is found on the X chromosome.

X-Linked traits • Found on the X chromosome • Show more in males; WHY? • Remember…♀=XX ♂=XY • Examples: 1. colorblindness a. 8% of males (are you?)

X-Linked traits (cont’d) 2. hemophilia: almost exclusively in males (blood cannot clot properly) 3. Duchenne muscular dystrophy (weakens and destroys muscle tissue; scoliosis, enlarged calves)

Section 2 Human Genetics Chapter 12 Genetic Traits and Disorders, continued • Sex-influenced Trait • A sex-influenced trait, such as pattern baldness, is expressed differently in men than in women even if it is on an autosome and both sexes have the same genotype. Example: Baldness -B dominant in males, recessive in females! - BB men…lose hair quickly!! - BB women…lose hair, but slower

Disorders due to nondisjunction • Nondisjunction- error during meiosis (gamete development); gamete can have extra chromosome, or NONE at all! • Monosomy: only 45 chromosomes! • Normal: 46 chromosomes • Trisomy: has 47 chromosomes! ex: Down’s syndrome

Down’s Syndrome

RELATIONSHIP OF DOWN SYNDROME INCIDENCE TO MOTHERS' AGE

Nondisjunction can also affect the sex chromosomes (X and Y) • Males with extra X (XXY) have Klinefelter’s Syndrome (feminine characteristics, mentally retarded, infertile)

Chapter 12 Detecting Genetic Disease • Genetic screening examines a person’s genetic makeup and potential risks of passing disorders to offspring. • Amniocentesis and chorionic villi sampling help physicians test a fetus for the presence of genetic disorders. • Genetic Counseling • Genetic counseling informs screened individuals about problems that might affect their offspring.

Chapter 12 Treating Genetic Disease • Genetic disorders are treated in various ways. • Among the treatments are symptom-relieving treatments and symptom-prevention measures, such as insulin injections for diabetes. • Gene Therapy • In gene therapy, a defective gene is replaced with a copy of a healthy gene. • Somatic cell gene therapy alters only body cells. • Germ cell gene therapy attempts to alter eggs or sperm.

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