<< Chapter < Page Chapter >> Page >

In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous.

Human sex-linked disorders

Sex-linkage studies in Morgan’s laboratory provided the fundamentals for understanding X-linked recessive disorders in humans, which include red-green color blindness, and Types A and B hemophilia. Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their sons, resulting in the son exhibiting the trait, or they can contribute the recessive allele to their daughters, resulting in the daughters being carriers of the trait ( [link] ). Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations.

A diagram shows an unaffected father with a dominant allele and an unaffected carrier mother with an x-linked recessive allele. Four figures of offspring are shown representing the various resulting genetic combinations: unaffected son, unaffected daughter, affected son, and unaffected carrier daughter.
The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A daughter will not be affected, but she will have a 50 percent chance of being a carrier like her mother.

Watch this video to learn more about sex-linked traits.

Lethality

A large proportion of genes in an individual’s genome are essential for survival. Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to sustain life and is therefore considered to be dominant over the nonfunctional allele. However, consider two heterozygous parents that have a genotype of wild-type/nonfunctional mutant for a hypothetical essential gene. In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die in utero , or die later in life, depending on what life stage requires this gene. An inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype is referred to as recessive lethal    .

For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed. The genotypic ratio would therefore be 2:1. In other instances, the recessive lethal allele might also exhibit a dominant (but not lethal) phenotype in the heterozygote. For instance, the recessive lethal Curly allele in Drosophila affects wing shape in the heterozygote form but is lethal in the homozygote.

Questions & Answers

what is biology
lilian Reply
what is biology
Dada Reply
The scientific study of life.
juanita
the virus that causes mumps in humans is composed of a protein outer Shell containing a core of DNA
Daniel Reply
Basic science and applied science question about cancer
Joyce Reply
what are the importance of ATPs
Olatunji Reply
How can biology be studied from a microscopic approach to a global approach
Joyce Reply
The large central opening in the poriferan body is called
Chynna Reply
You go for a long walk on a hot day. Give an example of a way in which homeostasis keeps your body healthy.
Joyce Reply
You sweat.
juanita
sweating is your bodies way of keeping you from overheating.
juanita
Thank you
Joyce
what is biology
Neya Reply
biology is the study of life
IYANUYIMIKA
Biology is the study of Life
Brianna
is the branch of science which deals with the of living things.
Peter
what is metabolism
Sangam Reply
hi palz, what's the ?
Gbolajobi Reply
hi plz, what's the.
Atta
pls come a again
Gyamfi
Describe the steps and results of reintroducing wolves to Yellowstone National Park.
Natalia Reply
tag and release wolves into Yellowstone. wolves eventually reproduce and the pack grows. as wolves hunt they cull the sick and weak prey. the carcass that is left provides food for other species (scavengers and insect.. etc). this heals the circle of life and contributes to the biodiversity...
Will
before you know it species that are critical to the eco system return. having apex predators is crucial to an ecosystem... it helps run the deer and elk , etc around.
Will
example: there was a species of shrub/ plant that grows along river banks that moose love to eat.. the moose have no predator so they decimate that food source which also helps prevent erosion. when the wolves were reintroduced this changed. oddly enough this plant species started to repopulate in
Will
the areas where wolf feces sat and decayed
Will
which of the following statements about the parts of an egg are false?
Israel Reply
Monotremes include...?
Israel
medicinal plants including microbs
vijay Reply
,medicinal plants including microbes
vijay
what is biology
Siyanbola Reply
study of living organisms...
R0se
study of plants and animals
Divine

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask