Chapter 14 Section 1: Human ChromosomesIf you had to pick an ideal organism for the study of genetics, would you choose one that produced lots of offspring, was easy to grow in the lab, and had a short life span that allowed you to do several crosses per month? You certainly would not choose an organism that produced very few offspring, had a long life span, and could not be grown in a lab. Yet, when we study human genetics, this is exactly the sort of organism we deal with.Given all of these difficulties, it may seem a wonder that we know as much about human genetics as we do.What is a karyotype?A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.To find what makes us uniquely human, we have to explore the human genome.A genome is the full set of genetic information that an organism carries in its DNA.A study of any genome starts with chromosomes, the bundles of DNA and protein found in the nuclei of eukaryotic cells. To see human chromosomes clearly, cell biologists photograph cells in mitosis, when the chromosomes are fully condensed and easy to view. Scientists then cut out the chromosomes from the photographs and arrange them in a picture known as a karyotype. It shows the complete diploid set of chromosomes groupedtogether in pairs, arranged in order of decreasing size.A karyotype from a typical human cell, which contains 46 chromosomes, is arranged in 23 pairs. Two of the 46 chromosomes in the human genome are known as sex chromosomes,because they determine an individual’s sex.Females have two copies of the X chromosome.Males have one X chromosome and one Y chromosome. This Punnett square illustrates why males and females are born in a roughly 50 : 50 ratio.All human egg cells carry a single X chromosome (23,X).However, half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y).This ensures that just about half the zygotes will be males and half will be females.More than 1200 genes are found on the X chromosome, some of which are shown.The human Y chromosome is much smaller than the X chromosome and contains onlyabout 140 genes, most of which are associated with male sex determination and sperm development.The remaining 44 human chromosomes are known as autosomal chromosomes, or autosomes.The complete human genome consists of 46 chromosomes, including 44 autosomes and 2 sex chromosomes.To quickly summarize the total number of chromosomes present in a human cell, biologists write 46,XX for females and 46,XY for males.What patterns of inheritance do human traits follow?
Many human traits follow a pattern of simple dominance.The alleles for many human genes display codominant inheritance.Because the X and Y chromosomes determine sex, the genes located on them show apattern of inheritance called sex-linked.