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

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section 14 1 review

Section 14.1& Review

Punnett Squares

Dihybrid Crosses



section 14 1 human chromosomes
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
i karyotypes
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
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
a sex chromosomes
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
b autosomal chromosomes
B. Autosomal Chromosomes
  • Remaining 44 chromosomes = autosomal chromosomes or autosomes
  • Complete human genome consists of 46 chromosomes = 44 autosomes + 2 sex chromosomes
ii transmission of human traits
II. Transmission of Human Traits
  • Human genes follow the same Mendelian patterns of inheritance as the genes of other organisms
a dominant and recessive alleles
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
b codominant and multiple alleles
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
c sex linked inheritance
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


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

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.

d x chromosome inactivation
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
iii human pedigrees
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
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
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
scenario 1
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.