What genetic criteria distinguish a case of extranuclear inheritance from: (b) a case of X-linked inheritance?

Verified step by step guidance

Step 1: Define extranuclear inheritance as the transmission of genetic material that is not located within the nucleus, typically involving organelles like mitochondria or chloroplasts, which have their own DNA and are usually inherited maternally.

Step 2: Define X-linked inheritance as the transmission of genes located on the X chromosome, which is one of the sex chromosomes, leading to distinct inheritance patterns based on the sex of the offspring.

Step 3: Identify that extranuclear inheritance is characterized by non-Mendelian patterns such as uniparental (usually maternal) inheritance, lack of segregation according to Mendel's laws, and absence of recombination between organelle genomes and nuclear genes.

Step 4: Contrast this with X-linked inheritance, which follows Mendelian principles but shows sex-specific patterns, such as males being hemizygous for X-linked genes and females being homozygous or heterozygous, leading to different phenotypic ratios in males and females.

Step 5: Summarize the key genetic criteria distinguishing the two: extranuclear inheritance involves cytoplasmic genes with maternal inheritance and no chromosomal segregation, whereas X-linked inheritance involves nuclear genes on the X chromosome with sex-dependent segregation and recombination.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Extranuclear Inheritance

Extranuclear inheritance refers to the transmission of genetic material located outside the nucleus, primarily in organelles like mitochondria and chloroplasts. This type of inheritance is typically maternal because these organelles are usually inherited from the egg cell. Traits governed by extranuclear genes do not follow Mendelian patterns and often show non-Mendelian inheritance.

X-linked Inheritance

X-linked inheritance involves genes located on the X chromosome, one of the two sex chromosomes. Because males have one X and one Y chromosome, recessive mutations on the X chromosome often manifest in males, while females may be carriers. This inheritance pattern shows distinct sex-linked traits and follows Mendelian principles but with sex-specific expression.

Genetic Criteria for Distinguishing Inheritance Patterns

Distinguishing extranuclear from X-linked inheritance involves analyzing inheritance patterns: extranuclear traits are maternally inherited regardless of sex, while X-linked traits show sex-specific patterns with males more frequently affected. Additionally, extranuclear inheritance does not involve chromosomal segregation, whereas X-linked traits follow chromosomal segregation and recombination.

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Textbook Question

Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 is afflicted with hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a normal son, despite the fact that the father has hemophilia. Several years later, couple 1 sues the hospital, claiming that these two newborns were swapped in the nursery following their birth. As a genetic counselor, you are called to testify. What information can you provide the jury concerning the allegation?

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Textbook Question

In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of F₁ cattle were mated to one another, the following F₂ data were obtained:

3/8 polled Dexters

3/8 polled Kerrys

1/8 horned Dexters

1/8 horned Kerrys

A geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned that Kerrys were true breeding. Dexters, on the other hand, were not true breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.

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