Section 14.1 & Review

1. Section 14.1& Review Punnett Squares Dihybrid Crosses Karyotypes Pedigrees

2. Section 14.1 – Human Chromosomes • Objectives • What is a karyotype? • What patterns of inheritance do human traits follow? • How can pedigrees be used to analyze human inheritance? • Define • Genome • Karyotype • Sex chromosomes • Autosome • Sex-linked gene • Pedigree

3. I. Karyotypes • What is a karyotype? • Human cells look like cells of other animals • To find uniqueness, must look deeper into genetic instructions that build each new individual • Genome – full set of genetic information that an organism carries in its DNA • Must look at chromosomes (bundles of DNA and protein) by photographing cells in mitosis when chromosomes are fully condensed and easy to view • Cut out chromosomes from photographs and arrange them in a picture • Karyotype – shows complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size

4. Typical human karyotype contains 46 chromosomes (arranged in 23 pairs) • We begin life when a haploid sperm (carrying 23 chromosomes) fertilizes a haploid egg (with 23 chromosomes) • The resulting diploid cell develops into a new individual and carries the full complement of 46 chromosomes

5. A. Sex Chromosomes • 2 of 46 chromosomes • Determine an individual’s sex • Females: 2 copies of X chromosome • Males: 1X and 1Y chromsome • Punnett square for male to female ratio!! • All egg cells carry a single X chromosomes • Half of all sperm cells carry and X chromosome and half carry a Y chromosome • Ensures about half zygotes will be males and half will be females • More than 1200 genes are found on the X chromosome • Y chromosome is much smaller & contains about 140 genes (most of which are associated with male sex determination and sperm development

6. B. Autosomal Chromosomes • Remaining 44 chromosomes = autosomal chromosomes or autosomes • Complete human genome consists of 46 chromosomes = 44 autosomes + 2 sex chromosomes

7. II. Transmission of Human Traits • Human genes follow the same Mendelian patterns of inheritance as the genes of other organisms

8. A. Dominant and Recessive Alleles • Many human traits follow a pattern of simple dominance • Hair color determined by MC1Rs • Red hair = 2 recessive alleles (one from each parent) • Dominant alleles produce darker hair color • Rhesus (Rh blood group) • Allele comes in 2 forms Rh+ (dominant) and Rh- (recessive) • Heterozygous (Rh+/Rh-) = Rh positive blood • Homozygous recessive (Rh-/Rh-) = Rh negative blood

9. B. Codominant and Multiple Alleles • The alleles for many human genes display codominant inheritance • Example: ABO blood group – determined by a gene with 3 alleles: IA, IB, and i • Alleles IA & IB are codominant • They produce molecules know as antigens on the surface of red blood cells • Individuals with IA& IB alleles  produce both antigens  blood type = AB • i allele is recessive • Individuals w/ IA IAor IA i only produce the A antigen  blood type A • Individuals w/ IB IBor IB i only produce the B antigen  blood type B • Homozygous for i allele (ii)  produce no antigen  blood type O • If a patient has AB-negative blood  has IA& IB alleles from ABO gene and 2 Rh- alleles from Rh gene

10. C. Sex-Linked Inheritance • Because the X and y chromosomes determine sex, the genes located on them show a pattern of inheritance called sex-linkage • Sex-linked gene – gene located on a sex chromosome • Genes on Y chromosome are found only in males and are passed directly from father to son `

11. Genes on X chromosome are found in both sexes • Men have just one X chromosome leads to some interesting consequences • Example: 3 genes for color vision (all located on X) • Males = defective allele for any of these genes results in colorblindness • Red/green = most common = 1/12 males • females = 1 in 200 affected b/c in order to be expressed in females  must be present in 2 copies (one on each X) • The recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females

12. If a woman is a carrier of an X-linked recessiveallele for a disorder and her mate does not haveit, their boys will have a 50% chance of inheritingthe disorder.  None of their girls will have it, buthalf of them are likely to be carriers. If a man has an X-linked recessive disorder and hismate does not carry the allele for it, all of their girlswill be carriers. None of their boys will inherit theharmful allele.  Only girls receive X chromosomesfrom their fathers.

13. D. X-Chromosome Inactivation • In female cells, most of the genes in one of the X chromosomes are randomly switched off  form dense region in nucleus (Barr Body) • Not found in males because their single X chromosome is still active • Example: Cat coat color gene is on X • One allele = orange • One allele = black • In cells in some parts of the body  one chromosome is switched off • In other parts of the body  the other X chromosome is switched off • Results – mixture of orange and black spots • Male cats (1 X chromosome) can have spots of only one color • If the cat’s fur has 3 colors (white with orange and black spots)  you can almost be certain that the cat is female

14. III. Human Pedigrees • To determine whether a trait is caused by a dominant or recessive allele; whether the gene for that trait is autosomal or sex-linked • Pedigree – chart that shows the relationships within a family – to analyze the pattern of inheritance followed by a particular trait • Shows the presence or absence of a trait according to the relationships between parents, siblings, and offspring • Can be used for any species • Example: white lock of hair just above forehead = trait • Passes through 3 generations of a family • Allele for white forelock trait is dominant • Grandfather had white forelock  passed to 2 of his children 3 grandchildren have the trait but 2 do not

16. Analyze pedigree  can often infer genotypes of family members • White forelock is dominant trait: all family members that do not have the trait must have homozygous recessive genotype • One of grandfather’s children lacks the trait  grandfather must be heterozygous for the trait

17. Information gained from pedigree analysis makes it possible to determine the nature of genes and alleles associated with inherited human traits • Can determine if allele for a trait is dominant or recessive; autosomal or sex-linked

18. Example #1

19. Example #2

20. Example #3

21. Example #4

22. Scenario #1 • Dimples in the cheeks are inherited as a dominant trait on an autosome. Using the proper form and symbols, draw a pedigree chart, beginning with a heterozygous, dimpled father (Dd), and a nondimpled mother (dd). Show four children of the expected types: boys, girls, dimples, and no dimples. Label your pedigree with phenotypes and genotypes.